Fare classes, IMO, are a better indicator of your odds of clearing. For 747 flights, If UA is showing Z9 ZN9 P9 PN9, then chances of R space being released is more likely, however if any of the four numbers are showing 4 or 5 or less, then your odds are pretty low.

For 757s, I think if P is 6 or higher then there is a decent chance of UA releasing R space for upgrades.

But then again, it all depends on how many people are waitlisted and where you stand on the waitlist. They may actually release a few R spaces in advance, but there might be people on the list that have higher priority than you.

Seat maps do give you a broad rough gauge of advance bookings, but that is the extent of it. Keep in mind, the C or F cabin booking will always be equal or greater number than the number of seats showing as occupied. Let's say there are 15 seats showing as occupied, then that means 15 or more passengers are booked in the C cabin. AFAIK, UA does not block C/F seats for no reason, and if they do it will be a crew rest seat.