I'd have to agree with effie here, again from someone who isn't a pro at neuropharmacology.
I'd say at most if the MAOI is still affecting your system at night the most you'd get, if anything, would be an increase in the duration of effects of the melatonin, which are weak already.
Taking large doses of melatonin doesn't really hurt you; not that its recommended. So I can't see these two drugs really being harmful together. I'd honestly guess at there being no interaction... or no noticeable interaction.
I really think there's no serotonin syndrome like effects to be worried about but perhaps someone more knowledgeable can chime in.