I don't think it's a stretch to say that Solutreans are "thought to be" mostly haplogroup I, even if academic papers won't usually engage in that sort of speculation. No tests on so ancient a sample have been conducted, obviously, but Maciamo, for example, has placed proto-I as most likely in Europe by the Upper Paleolithic:
http://www.eupedia.com/forum/threads/29080-Chronology-of-Cro-Magnon-Y-DNA
I've read similar sentiments from several other prominent hobbyists, but admittedly little from academics, who have tended to be either silent or wrong. So we still need ancient samples, as always, but in terms of speculation, placing the Solutreans as some sort of mix of Haplogroup I and Haplogroup C seems to be the most reasonable guess based on what we know at the moment, if we assign them to any extant haplogroup at all.