I suspect that mellotrons do or at least could have aftertouch in the sense of the sound being altered slightly by pressure on the keyboard after the tape starts to play. I have read of concert pianists talking about aftertouch on concert grand pianos although my chinese upright doesn't seem to have it.
On M-tron pro it would be really useful as one could introduce another sample, change tone color, panning,delay chorus etc. However my question was just curiosity as I don't have any keyboards with aftertouch.