A couple of things I discovered tonight is. Firstly you need to use vst3 versions in irder to use third party plug-ins for sidechaining key inputs

In the plug-in properties for bitbridge if it has 4 channel input enabled then stereo sidechaining is possible.

This might be obvious to some but I like discovering this sort of thing without having to ask.

Ok the answer is, I just need a dynamic EQ/compressor. I tried using Wave F6 Dynamic EQ which works to a point. For example, you could use 8kHz as a central point. A really wide Q value of 0.5 and set a reduction in the area. The problem for me I suppose now is what do I use to trigger the sidechain or do I just use it on the master bus as is which in theory should do the same thing as long as I change the mix value to make it parallel mixing.

I'm sure this is too simple.. and I'm missing something..how about create a BP for all the treble range you want to leave alone, then compress the rest, and add back the uncompressed range?

I saw the trick in a Reason video, where the guy set up a compressor for the whole bus (mixer) and the whole mix gets compressed there however a sidechain EQ on that compressor uses an external EQ to only change those frequencies.
 
I'm pretty sure I over thought it though. Let me think about it and I'll come back with an answer.
 

I have mastered quite a few albums and from my experiences I have found if the mix is nicely balanced across the spectrum then in many cases you don't need to do this and in fact a single full range compressor can work very well. And with no side chain filtering taking place either.  I often use the API2500 for this job or some of the Tokyo Dawn Labs compressors as well.  Even when I was using outboard hardware compressors such as the Smart C2 I never had to side chain either.  It really all comes back to making that mix well balanced.  And don't forget that compressor over a mix need not be working too hard either.  Like 1.5:1 ratio with 2 to 3 dB's of GR. So under these circumstances using the side chain is also not really necessary.  I have sat in with some top mastering engineers and they did not side chain things either.
 
In situations where I had difficult mixes to compress, I found the multiband to be a bit of a saviour here. You can set it too to not be doing much across the spectrum e.g. low ratio and low amounts of GR but when a band comes along that is excessive for whatever reason then it trips the compressor harder and keeps things in line.  Both the Cakewalk and Studio One multibands are excellent although in some ways I find setting the Cakewalk one a little easier. The Studio One GUI is almost a little too complicated but once you know it then it does work well too. 

It's a trick really for an effect more than anything. For certain tracks that are EDM although I know essentially it won't sound right on everything. I will rewatch that reason video and see of I can work something out.

interpolated
It's a trick really for an effect more than anything. For certain tracks that are EDM although I know essentially it won't sound right on everything. I will rewatch that reason video and see of I can work something out.

 
Yes of course. Your OP did not mention anything about genre.  I am leaning to the pristine much more transparent approach to compression in mastering.  But for EDM and other styles then for sure you may need some serious side chain options in order to get the effect you are after.  I have not mastered an album that has called for it as yet.  But I am interested in how you may achieve it though.

I may need to set up an auxiliary track which all the mix busses get sent to which in then receives the sidechain information from fx bus before summing it all through the master channel. The master bus only seems to trigger through prochannel and cakewalk effects.