While you might be able to power up with USB 5V power, the voltage is just above the M100's threshold of approximately 4.9 volts. And the amperage isn't likely to be sufficient for some of the more taxing tasks, such as powering the bar-code reader.
All in all I would second the unit recommended by Georg Kaeter; I have a similar RS unit (in addition to two Tandy wall warts, which are formally designated as "TRS-80 AC ADAPTER, CAT.NO. 26-3804 FOR USE WITH TRS 80 MODEL 100 COMPUTERS. INPUT 120V AC OUTPUT 6V DC.") -CH-
