I've never asked. But I have been stopped either by the scanner or manually (sometimes even after the scanner has said OK) and given a new C/F BP many times on various airlines (including UA as 1P). Sometimes for obvious reasons (they need seats etc). Occasionally for no obvious reason (eg economy still half empty, but maybe a GA feeling like spreading goodwill amongst low (1P) elites ...)

Ignore the possibility until it happens - never hope or expect it - then it's a surprise when it happens, and can make your day. Having said that, I've had it happen often enough (probably better than 1 in 10).

But once on board, your chances are vanishingly small, as it's much easier for them to op-up someone still waiting to board than come find you. Unless its an on-board op-up after the doors close to sort out a problem (eg broken seat).