Could either the installer or the MIDI plugin have the ability to pull down alternative DLS files ?
Rationale:
In the early days of Winamp there was more chance that people were using a standalone sound card (very likely from Creative Labs) that came with better instruments, or that users could go to the home page of their sound card and download them.
Now most people are using onboard sound, so stuck with the basic version in Windows.
Alternatively how about just including some useful links in the plugin ?
(This at least is a short term solution that could be done now)