Patch D67 Mellowtron on the JV1000 synthesizer creates a fairly convincing mellotron patch using just factory waveforms. This patch can be ported over to any of the JV/XP series synths and is available online along with all the other JV1000 presets at http://www.wsd.net.au/jvxp/
If you only want the mellotron patch, just download the D bank.