I'm looking for yaoi series with different sex positions than the regular old front (uke's legs wrapped around the seme or over the seme's shoulders) and back (doggy style). Are there any with other...more unique positions?

My best responds is yes.. but they are harder to come by. It's the same as hentai (the weirder the position the harder to find). But you can find them if you really look.

Plus I think your forgetting the other "normal" ones, like:
69
other oral sex (there are many positions)
cowgirl/boy
sitting (aka cowgirl with "seme" sitting up instead of laying down)
spooning

Haru wo Daite Ita does quite a few "other" positions. The most "unique" is is when Kato is holding Iwaki is a standing position with out support of any kind (aka Iwaki is not touching the ground/wall/ceiling/tree...) volume 7.