two things about the mellotron to remember:

one is that it is tapes of real instruments, played back out of tune
and in very poor fidelity

the second is that the original mellotron tapes used through the 60s
and 70s are SO familiar from all the records we hear them on.

the second point really needs the proper Vintage ROM as the samples
are dervived from a real tron.

but the first point is easy enough - take any flute / string / choir
sample and filter it way down (apparently the 'tron had a bandwisth
of 8Khz!), then put a random lfo through a lag genertor to
constantly vary the pitch only 'just' detectably, then take one of
the random generators and add to the initial pitch, so that every
note starts slightly differently... again, only 'just' detectably.

post how you get on!