I had luck on my very first flight eligible for an upgrade... a midnight departure from LAS to OAK. The flight was fairly full but First had enough availability even for a lowly Silver.

I'll be departing on a late-night Wednesday to red-eye itinerary OAK-LAS-RDU, returning Monday afternoon/evening RDU-PHX-LAS... and will be sure to report my upgrade success (or lack thereof) after I return.

My sense is that red-eyes might not be the best... the late-night HP flights I've been on so far have all been fairly full in Y. However, perhaps the population of business travelers likely to be Elite is less?