This is a nice effective little trick I watched on Mixbus TV on YouTube. Now all I need is a MIDI triggerable upward compressor, lol...
 

Off the top of my head, using an expander side-chained to a drum sample, with the drum sample track either muted or with the track fader all the way down.

Or if the notes have clear separation in time a transient shaper might work.

Or easier still, just tweak the synth to have more attack, lol!

Ah, but when the accordion is a real accordion, not what someone in Japan thinks an accordion might sound like to someone who cant tell their accordian from their diatonic accordion from their concertina or bandoneon or harmonica, then the attack the instrument/musician has is the attack you're gonna get.

Some of the free reed+bellows family are very punchy, others are naturally smooth like a reed organ.

Therefore transient shaping, compressor/expander side-chain controlled pumping etc. No different to handling a heavy guitar riff that's been tracked so distorted and squished it lacks clear dynamics really. Better to record it right in the first place, of course.

Sharke, could this be what you are looking for?
 

Real ones sometimes have clacky key noises in my experience. The expander would likely make this mechanical noise worse.

That is for acoustic recordings.

Very true. I've a very nice Cajun-style melodeon that clacks quite badly. Close micing makes the problem really obvious, but live it's OK like a humming amp is often OK live. It's recording where the noise becomes objectionable.

Soundwise
Sharke, could this be what you are looking for?
 

 

That would do it at a uniform speed though instead of on a per note basis? 

I can think of several approaches I'd try first, before resorting to MIDI-controlled expansion. 
 
One of the easiest (but least versatile) is a tempo-synced tremolo. Meldaproduction has a couple of syncable tremolos, with wildly adjustable modulator waveforms. You can, for example, pump the full mix to lend additional rhythmic punctuation. And it's easy to adjust from very subtle to in-yer-face, or to modulate that adjustment via automation. This technique is especially applicable to the scenario described in the video.
 
In the example of a too-soft attack on an sampled instrument, I'd be inclined to solve that by layering. Clone the track and route it to a soft synth, and then design any attack you want. Use a short pluck patch to make your acoustic piano sound like a tack piano, for example. Make the pluck's volume and filter velocity-sensitive so you're only boosting notes that were already accented to begin with. 
 
Going up in complexity, an envelope follower lets you modify any portion of an instrument's envelope. FabFilter Saturn is a great example, as its modulation features are quite extensive. I use it a lot on bass if it needs to be punchier, by modulating both volume and distortion. You can also punch leads up with FabFilter Timeless, or really any delay/reverb that offers a ducking feature. Smudging the release but not the attack has the effect of bringing out the instrument's attack, in the way that dipping one EQ band yields the same result as raising a complementary band. Boz Millar's Imperial Delay is especially good for this.
 
Probably the last resort (next to MIDI-controlled expansion) would be a transient shaper. It's always hit-and-miss with transient designers, but it's easy to stick one in and twiddle its two knobs to see if it's going to work. Multi-band transient shapers can be very cool for pumping up acoustic drum tracks when all you have to work with is pre-mixed audio.
 
But if you really like the technique shown in the video, the Sonitus compressor and Sonitus gate can do it. No need to buy Pro-G.
 

bitflipper
One of the easiest (but least versatile) is a tempo-synced tremolo. Meldaproduction has a couple of syncable tremolos, with wildly adjustable modulator waveforms. You can, for example, pump the full mix to lend additional rhythmic punctuation. And it's easy to adjust from very subtle to in-yer-face, or to modulate that adjustment via automation. This technique is especially applicable to the scenario described in the video.

 
It would be, if the lead consisted of uniform notes. However I think the whole purpose of doing it with MIDI notes is that you can place the notes wherever you like, to match a non-uniform lead melody. 
 

bitflipper
Going up in complexity, an envelope follower lets you modify any portion of an instrument's envelope. FabFilter Saturn is a great example, as its modulation features are quite extensive. I use it a lot on bass if it needs to be punchier, by modulating both volume and distortion. You can also punch leads up with FabFilter Timeless, or really any delay/reverb that offers a ducking feature. Smudging the release but not the attack has the effect of bringing out the instrument's attack, in the way that dipping one EQ band yields the same result as raising a complementary band. Boz Millar's Imperial Delay is especially good for this.
 
Probably the last resort (next to MIDI-controlled expansion) would be a transient shaper. It's always hit-and-miss with transient designers, but it's easy to stick one in and twiddle its two knobs to see if it's going to work. Multi-band transient shapers can be very cool for pumping up acoustic drum tracks when all you have to work with is pre-mixed audio.

Someone mentions transient shapers in the comments and like he replies, a transient shaper is good if the transient is there to begin with, but in some cases (like the track in the video), there's not much of a transient to work with. I think an envelope follower would work though, you could probably do it in Guitar Rig.