Thanks for the info woodpecka. I think I should be good without a MOST-enabled HU - my BMW Assist has expired and I'm not planning to renew it. And with this particular HU, I believe my existing TCU/MULF will be redundant, since the mic and USB inputs are connected directly to the HU.
Unless I'm looking at this the wrong way (and if I am, someone please let me know!), then I think should be able to do without a MOST-enabled HU, except for when I have to get a dealer-flashed software update. If that scenario arises, I could always put the old HU back temporarily.