Hi Andy,
A quick update how it goes with NI ARD and Roland TD-20X:
I tried to set it up various ways (using TD-9/TD-20 preset vs default preset) and soon had to discover that
(a) the TD-20 preset doesn't properly map the factory settings of the TD-20
(b) you cannot create user presets in all ARD modules (save of customization doesn't work in Modern Drummer)
(c) you would need a different set-up in each ARD module due to different drums (e.g. 2 toms vs. 4 toms, ...)
However, using a Sonar drum map the remapping is done once and fits all ARD mods; plus that allows me to switch between using ARD or TD-20X sounds for the same MIDI drum recording just by swapping the drum map. Quite nice once set-up.
In terms of using ARD as sound source while recording MIDI drum tracks, I initially had problems that some hits were swallowed (some just weren't played although it had no relation to where I hit the pad), but when I looked at the recorded MIDI track I realized that there are low velocity MIDI notes from faulty triggering (i.e. hitting a tom causes very low velocity cymbal notes to play along). While the TD-20X can cope with that (you only get a very faint cymbal sound that's only audible if toms are muted), ARD didn't seem to take that so well. So I spent about half an hour to RTFM and optimize trigger options inside the TD-20X to get rid of these low velocity notes and now ARD seems to play all hits and can handle a dynamic groove with ghost notes quite well. Latency with a beefy DAW is not an issue; it's pretty playable

as long as I remember to keep playing in the center of the snare pad (ARD of course doesn't understand the positional sensing of the TD-20X)
So bottomline is: it's worth to go into the trigger / monitoring options of your module and tune that to fit the pads your using (I'm using custom ones from a German company instead of original Roland).
The only remaining issue for the time being is that cymbal choking with the default preset works only for the Ride cymbal but not for crash cymbals or AUX cymbals (which I had to remap in the drum map). It may be that choking (MIDI aftertouch?) messages are still send to the original note and not remapped. Maybe you give me a hint what I could try to sort that out? It's not overly critical but it would be nice to get sorted. At the moment I tend to like the TD-20 cymbal more, but that comes from playing the kit solo and may change when mixing the next session ...