Although it's not exactly what you're looking for, I'd have to second the Nikon Series E lenses. They truly are remarkable little lenses. I haven't been able to find a good copy of the 35mm 2.5 but the 36-72, 50mm and 100mm are nice little lenses, especially the size of the 100mm. Also the older non-ai lenses are pretty spectacular as well. One lens I hate to mention because it does get such bad reviews is the 35-70 3.3-4.5 ai-s lens. It's tiny and a lot better than the bad reviews give it. I mean, it isn't great, but there's something very Nikon in its not greatness.
Another zoom lens I love, which I assume you're familiar with is the Canon FD 35-105mm 3.5. This lens is ALMOST parfocal. Obviously constant aperture. Internal zoom. Fairly compact. I've had a few copies over the years, then end up selling it because I didn't use it too often, regret it and seek another one. My most recent copy I put on the Sigma FP and here's a sample of it at 105mm wide open...
No mind-blowing image by any means, but fairly sharp wide open and I like the way it handles the highlights. Of course it's a mess in direct light... as I found out last week during a shoot. However, there are rumors that Canon considered putting the L badge onto it because they were so happy with the lens.
As far as primes, other than the Nikons, the Takumar lenses are a personal favorite of mine. I've heard people argue that the 28mm 3.5 is one of the best 28mm lenses they've used. I haven't used that one but the 35mm 3.5 is excellent.
I've used a couple of the wide angle Tokina lenses from the late 70s, early 80s and they were decent, but I think they were f/2.8 lenses. Obviously you could get a cheap Neewer fixed ND to pop on the lens and leave there to give you that slower stop and to protect the front of the lens. Also check out the old Vivitar lenses, I have a set of mostly Tokina made ones in the m42 mount and I have always been very fond of them. The image instantly feels like an old 70s made for TV movie. I'm probably going to sell them though, not because I don't like them, I just have too much stuff and really need to scale back.
I guess you could also look at some of the older Zeiss Jena lenses... even the MF ones would give you the slower apertures you were looking for wide open.
Great thread! Wish there were more threads like this on here like it.