I don’t know if this helps or muddies the issue, and I can’t answer for Pjotr, but I do know that in the 18th century (1741, specifically), a similar notation was used, and my editor and I are pretty sure it genuinely is a “common time” symbol followed by another time signature:
As best we can tell, it’s the exact same symbol he uses when there’s a normal common time:
So in our edition, we went with the actual common time symbol followed by 2/4 or 3/4, which could not have been more of a pain to deal with in Finale. It would be so lovely if at some point this was supported in Dorico.
As far as Finale’s ability to use an alternate time signature for display, it is a feature that I’ve used many times over, and I don’t know what I would’ve done without it. That said, my main use for it was, similarly, defining beat groupings (e.g. display 3+2+2/8 as 7/8), which Dorico does natively (and saves me just boatloads of time). Other than that, there’s the case of needing to remove a beat from a final measure when there was a pickup at the beginning of the piece or section (does Dorico support this, btw?), and maybe an odd edge case here and there.
Soo… I guess what I’m trying to say is that I love how Dorico has generally removed the need to display the “wrong” time signature, and that’s probably the better avenue to pursue for most use cases, but it would be nice to be able to get under the hood and tell Dorico to do “wrong” thing from time to time when those few edge cases do come up.