The 288 doesn't have a mute on the bass, it was designed as an acoustic accordion from day 1", but that in no way should interfere.
Personal opinion time (and that is all I can share... lol), if I was going to MIDI an accordion it would have both right and left hands done for the simple reason that one day if my tastes changed, I would have the option to have some low strings accompany my accordion's right hand. Also resale value is (slightly) higher if both hands are done versus just the left hand. I'd rather have it and have the choice to use it or not, over needing it and not being able to.
In the UK," left hand only MIDI" is more prevalent and popular, especially on the smaller accordions. For them I also figure its not just a choice but a cost/funds available situation.
When playing the Elka, I'd mostly play acoustic accordion on the right hand augmented by "electronic" bass on the left, but I would also often use organ, trumpets, strings as a background to my acoustic, it added a lot to the overall sound and sonic variety. I also added arrangers for a rhythm section and that was a big part of what became my style when performing. This kept the music more interesting and varied for me... anything that helped me create a more enjoyable experience for me and the audience was definitely a plus, but it was also having the options. I didn't always use an arranger, there were times I just flicked off all arranger/MIDI sounds and used the acoustic accordion all alone and strolled around for a song or 2.
Not everyone has my tastes and I definitely do not expect everyone to share my opinions either, but choices are a good thing... kind of like a condom or a gun... its nice to have if you need it, but not good when you need it and don't have it.
Accordions are not a life/death situation, so the above may not be completely appropriate, but it is my opinion.
In terms of bass modules, there are a few, but I want to share an opinion here too, Ed. You are not the kind of guy that is going to use 200 different bass sounds. I think you will choose 1-3 basic sounds and stick to those for the most part. You may likely not even use the sounds on the chords, so you don't need a BK-7m or a V3 arranger (though that could change if you really like a sound or tone that they make), instead look for smaller/cheaper modules and check f they have the 1-3 sounds that you like... you might even find that something "vintage" like an old Yamaha FB-01 module just might have those couple sounds that you like... so when shopping, listen to the sounds and see if whatever module you are reviewing fits your needs. You may not need that Ketron SD-90 (at $4500... lol), but may find some used unit for a hundred bucks or so.