I've been playing around with that Mellowtron/Yellotron pd patch, and putting my own samples in it. I've figured it out pretty good. I cobbled together a work around for velocity layers too, but could use somebody's help to make a better design.
I think @yeahsometimes might be in the same boat as me. It would be sweet to have a patch where we could easily drop multi-sampled sounds WITH multiple velocity layers. Does this exist yet?