I'm certain they won't do una corda since they have to re-record the entire pianos and it would effectively double the size of the download. Frankly I don't even like it on my Baldwin 5'8" grand because when the dampers come down they make a little bit of a twang because not all the strings are vibrating equally. It was that way when it was new. But the sustain resonances would be very simple for them to do.
I've been doing the sustain resonances myself using the sampler Kontakt. If anyone else wants to know how, let me know. It's a bit of work.
I agree true una corda samples are unlikely--but a better una corda model should be doable--and as I think someone else on this forum mentioned could be adjustable (maybe even including felt or moderator options as as alternatives to una corda effect).
Nevertheless, I think una corda samples should be considered, at least for a limited set of pianos and/or mics. The Steinway perhaps has a large enough market to support una corda as a paid upgrade or even separate alternative instrument. The 280VC might be another good candidate, on the assumption Stage B makes scheduling a new recording session easier.