Have you tried Waves Renaissance Reverberator (RVerb)? I've been using it (along with the other remarkable plugins in my Gold Bundle) for years now and am quite happy. RVerb's emulation of plate reverb is excellent for Mellotron - especially for 3 Violins, if you're looking for that Court/Epitaph/Starless sound.
I feel obliged to disclose the fact that I do not own an actual unit, but am using Pinder's CD with VSampler.
Regards,
Dagfinn