i definitely feel for you on the looping problem. i've tried to jam with guitarist friends several times when they want to use looping pedals and it just will not work. nobody's timing is that good. one option is to use a pedal with a "restart" feature for the loop (like the Boss Loopstation) and just hit that every so often when its starting to get off. but that's less than ideal, especially if you have other pedals you wanna work.
there is one looper that has MIDI - the Gibson Echoplex. it was originally made by Oberheim but then Gibson made them again recently. it's got a lot of sampling time, delay features and a pedal that allows you to stack and unstack loops one-by-one. they go for about $600-800 tho. but if you bought an MD-UW for drum accompaniment, then you're probably of the mindset that you get what you pay for/good gear don't come cheap. so...
yes, you can use Ableton live for similar features. but keep in mind, it's $400. plus you need a laptop ($1K+), a firewire sound card ($200+) and a footswitch to control it ($200+). so the Echoplex starts to look very promising, no?
and no, I don't think sampling the guitar into the UW is really an option. that would only give you two bars, whereas the Boomerang offers what, like several minutes of phrase sampling time.
