So I was chatting with my wife about this and she mentioned that last week she turned it in on her 86 paladin. I told her there was someone here that had their spouse try and couldn't. Since she hadn't done the tournament yet (and I had), I asked her if her paladin was still under 90. She was 87, so I asked her to try tonight. Sure enough, she was able to turn it in on her 87 paladin. I'm not sure what the cutoff is, but you don't have to be 90 to turn it in (she still accepted the quest and did it on her 90, she just turned it in on her 87). I had her take a screenshot. I'll go through and erase the names, etc for her and post it in a bit.
EDIT: image on tinypic.com:http://i59.tinypic.com/2gwid1c.jpg