I put about an hour plus into this and concluded I can not get it to work for me. That is not saying BIAB will not do its part, I am only saying I can not make it work out for me. I started this 6/8 project specifically to write in 6/8 as a learning exercise. And that objective is being met. What I have concluded so far is that:

1) I understand "most" of what Peter has said about how to count 6/8. And it is clear there is a ready association with 4/4 counted as triplets. I get that...
2) I discovered that designating 6/8 time signature in Reaper or in EZKeys makes absolutely no difference to what you hear, only to how the notation appears. The most immediatly noted difference the number of bars in the song. My demo song in Reaper and EZKeys is 95 bars when set to 6/8 but only 70 bars when set to 4/4. But again, sounds exactly the same.
3) I was able to bring the midi piano track from Reaper into BIAB. However, regardless of time signature, I was not able to duplicate the number of bars. BIAB only shows ~49 bars when 4/4 is specified. This was a serious problem in syncing when I brought any RTs back into Reaper. The track did not line up in time?
4) I do admit, the 90 tempo across Reaper or BIAB seemed to match (I was surprized).

Bottom line for me... I can't do it! Perhaps if I did the entire song from scratch in BIAB I could avoid the problems I had. But that is not my workflow. My chords come from other sources. I wish BIAB would allow cut and paste, cause I believe my problem maybe with the midi input with ACW. We have talked about our wish that BIAB would play better with, as Mario has stated, "the rest of the world". But that is another thread.

BIAB – 2025, Reaper (current), i7-12700F Processor, 32GB DDR4-3200MHz RAM, 1TB WD Black NVMe SSD, 2TB WDC Blue SSD, 1TB WD Blue, 2 TB SK NVMe, 6 TB External, Motu Audio Express 6x6