Unfortunately, we live in a new age and China has perfected facial recognition software, which they will probably have no scruples about providing to Russia. I'm not sure a crowd provides the same kind of anonymity that it once did.
But you're right, the conflict may or may not actually reach the stage of clandestine insurgency.
We are starting to see some of this right now (and I'm not 100% certain) but for instance the alleged Azov battalion operating near DNR/LNR area is said to be not allowing citizens to leave the city. There are footages going around them turning (you can tell from the patches they wear) citizens away from checkpoints as they try to leave.
Like Hamas in Palestine who regularly launch rockets at Israel from top of apartment buildings, it's only a matter of time before insurgency turns to this tactic as they are determined to keep fighting.
Do you suppose that may have something to do with things like
https://www.newsweek.com/evacuation-route-offered-fleeing-uk...
and
https://edition.cnn.com/2022/03/06/europe/ukraine-russia-inv...
?
Another side of the story is that Ukrainians claim they couldn't reach agreement of ceasing fire and humanitarian corridors with Russians.
I believe people are leaving Kharkov and Kiev relatively freely.