You can still route midi on different channels to a single plugin in ableton live, it just requires a whole lot of workaround routing. Basically, you create a midi track for each midi-channel, set it up so that it only takes input from that one channel and sends its output directly to the input corresponding to the channel of the plugin (e.g. reaktor). It's terrible if you want to record midi, because you have to arm all tracks at the same time and then get a midi clip for each channel, but it does work. And if you're fine with just recording your performance in audio, it's actually kind of usable if you set it up as a template.