I just called UA to ask, particularly based on the "new rule" discussed above. I mentioned my seven-hour layover in SIN.

An international reservationist at the 1K desk said I would have no problem. I then added that I had heard about a possible new limitation of four hours on the layover. She responded by stating she was looking at a screen that made it clear that UA would/should check my bags all the way from BOS to PEN.

When I asked what my course of action should be if UA happens to erroneously refuse to check the bags to PEN, she advised me to get to BOS early enough to ask UA to call for confirmation of the rule and, if there is still a problem, get an international rep at the 1K desk on the phone.