I haven't done it because I don't own one, Simp, but given the broad voltage range supported (5.5v - 25v) and the minimal current draw (50mA), it'll be a breeze to put together a compact, long-lasting external power source.
Personally, I think NiMH 9.6v RC batts would be a great way to go. Even a
small 1000 mAH RC batt will give you ~20 theoretical hours of recording time.
There are some pretty good instructions with pics in the Archival Forum:
http://www.taperssection.com/reference/html/DIY_EdirolUA5Power/http://home.att.net/~joemacjr/diyproject/soldering.htmlThe differences for your application will be:
[1] no real need to wire two batts in parallel since you'll get such ridiculously long run-time off a single battery
[2] instead of the adaptaplug connector, you'll use whatever connector and pinout the AD20 requires (mono 3.5mm male connector, tip (+), ring (-) if memory serves)
Curious to see if anyone's done this for the AD20 before...