Send MIDI note on with velocity 0

You are exactly correct, m4L has a limited midi implementation due to Live commandeering your messages when they leave the device. I too, have had some infuriating spirals into confusion and self loathing fun experiences running up against theses limitations, and (to my knowledge) I don't think there is a doc that spells them out fully. To my knowledge midi beat clock and midi time code are strictly forbidden and midi channels are stripped and replaced with whatever the track's channel is (so one track = one channel). The sysex implementation has recently gone from completely non existent to limited and obtuse, so that is good. I haven't had any trouble with nprns, so that is also good.

Any way one solution (my fav solution) is to have max vanilla running alongside Live, and you can communicate between the midi them with sends and receives. just don't use the '- - - ' (three dashes) before the send/receive name. Buffers can communicate in this way too, although less suited for this purpose imo. You can even make a standalone max app to receive the midi if you want to get super slick.

You can use a similar technique in live to get around the channel & sysex limitation, using a send device on one track and a receive device on another, excluding the '- - -'. There is a set of example devices in the 'Max 7 Pitch and Time Machines Pack' that illustrate this technique, one device that sends and one that receives. There is a little latency introduced, but i find it to be tolerable, as long as you are careful not to add latency elsewhere (such as with js).