You could use compact car audio type equipment powered off a 4 cell Lithium Ion pack, it will be heavy but still man portable. The hard part is discovering what amount of audio output power you'll need to get the loudness you require. One way to determine this would be to use a commercial amplifier of a known wattage, use a mic/amp to play your squeezebox through it, and turn the volume dial 1-10 up to the volume you think you're going to want to produce. The number on the dial divided by ten will give you a value between .1 (10 percent) and 1 (100%) of the total output power used, multiple that times the known wattage of the system you used and you'll have an approximation of how much power you'll need for the portable system.
Come back with that number and we might be able to help you better.
One problem is how you record this test will effect the results in the real world device, microphone placement is hyper critical and I'm not sure where the best location for a microphone is in your instrument, that would be something you'd need to research, there has to be sites out there that have tips on microphone placements for acoustical instruments.
For starters if you have the ability to do so, simply place the microphone (a simple condenser mic such as the typical PC uses would be fine) near the primary sound output of the instrument (but away from any air it's exausting) and start your test from there, that'll give you a ballpark. Proper placement of the microphone and the gain of the pre-amp are also pretty important, but one step at a time.