UPN does have secondary affiliates that air it's primetime programming in different slots. Most such stations are the primary affiliate of another network like WB or FOX, since both of those networks don't provide as much network programming as NBC, ABC, and CBS. This leaves more local primetime slots open for another networks programming.
The station I used to work at was a primary FOX secondary UPN affiliate. ST: Voyager was on Wednesday nights for primary affiliates, but we aired it on the following Saturday at 6p, FOX network primetime feed doesn't start until 7p. This was a few years ago, of course. Later we launched a second station to be a primary UPN affiliate.
I'm not sure that all made sense, but basically it boils down to the market. Most large markets will have a primary UPN affiliate and air their programming in the "standard" time slot (ie. Buffy is on Tuesdays 8/7). Most midsize markets will have a secondary UPN affiliate that will air some UPN programming (usually the most popular shows) at some later time in the week. Smaller markets will probably have any UPN affiliates at all, since UPN is still very much the small fish in the pond.
This is all different, of course, in Canada, Great Britain and other international markets. I'm not exactly sure how it works, but at least some of them have to wait until the show can be syndicated (about 7 months after first air date) before they get it. Someone may want to double-check me on this part, since I never really studied it.
sotmh