I have found that slowly and slightly modulating the Room Size parameter can sound very nice. I do this in Reaper with its parameter modulation functionality. The CPU use jumps up noticeably though. This is partly due to the Room Size parameter controlling many parameters at once, but it sounds richer than modulating only one parameter (such as Reverb Start).
I think it would be a great addition to have a modulation parameter built into Ircam Verb. Might this be possible?