Hmm, I started doing this about 60 turns in, but I got the "unreliable ally" penalty with my ally the Ouichi. Not quite sure when it appeared though.

I made peace with the Ikko for $, then went to the Ouichi and offered to help them against the Ikko for 1k. I don't think I've done anything else to annoy them. Have always joined them when they declared, and they have joined me when people have come after me.