A bit old i know, but i bumped into this thread by accident (quick search seems to show this as the latest one on this topic)
One thing you can do is install a web-control plug in for winamp then access it via the ingame browser. Gave it a try, quick google search gave me Ajaxamp - installed, configured, tried it - lo and behold it does what you want. There was some incompatibilities with the browser but i could load play lists, pause, skip etc. happily (jumping between tracks in a play list didn't work).
http://animemaho.com/xebec/eq2webmp3.jpg
There are plenty of such plug ins out there so a bit of searching will likely turn up something more suitable - heck writing one tailored to the eq2browser wouldn't be that bad.
(apologies if someone has already pointed this out, didn't see any threads related to it - but i could have searched deeper)