Tom,
I am interested in better understanding your last sentence, "A par-5 opener needs to be pretty dull or there is the possibility of making six to start, which is no fun."
Surely no one likes an initial bogey (and I've had my fair share) but the same can be said for any par-4. Why/how does making it par-5 make a difference whatsoever?
As you noted, George Thomas did a few (most notably LACC and Riviera) and they are rather benign "gentle handshakes."
I used to think along the same lines, because George Thomas's book made a strong impression on me, and as I say, I have built quite a few par-5 openers on my courses and never thought anything of it.
But, as an architect I have a hard time designing a boring green on a short par-5, and I found that a sloppy third shot would lead to a lot of 6's and even a few 7's, and who likes that right out of the gate? As a player whose goal was to break 80, keeping 6's off the scorecard was a key for me. So I just did not enjoy playing those holes as much as I anticipated.
The holes mentioned like the 1st at Sunningdale or LACC are good openers, but not the sort of hole I'd be likely to build. My favorite openers among my own courses are the short par-4 at Lost Dunes, and the medium par-4 at Old Macdonald,