Rubinek had a rather extensive recurring role in the middle-late seasons of "Fraiser" so he's been doing TV on a more regular basis for quite a while...
Also, Lawrence Pressmen (Ghemor) can't be on the list either. Ghemor was seen twice on DS9, but he also played a Starfleet Admiral at one point I think, and he was the Changeling Odo killed in the 3rd season finale.
Likewise, Frank Langella's role was for 2 or 3 episodes so... does it count as one-off?
I have read repeatedly that Langella did the role on DS9 specifically as a favor to his kids, who where big fans of Trek. They wanted him to do something on Trek, he did that, I don't think he was credited officially (by choice) for it and... that's all that's written about it.
I think I read an interview with O'Quinn in which he mentioned his TNG appearance. He was talking about how he had done it, and so had Daniel Dae Kim (this was during "Lost") and someone else had also and... it's a vague memory of it being mentioned.
Kelsey Grammer - it's reported he cell-phoned in a vocal cameo for "First Contact" during the distress sequence audio. He apparently said "Acknowledged" after some orders where issued to "Defiant and Bozeman, fall back to mobile position 10" or some such (I can't remember the exact line, even though I've seen the film 135 times) so would that break the one-time rule? :P
Sloyan would be interesting. He did the voice-overs for that one car company, Infinity maybe, for the better part of a decade, didn't he? He's impossible to find a decent photo of. I know, I looked, wanted to have him in an Ultimately Human photo special back in the day. Could not find anything.