Thanks for the info. I've got the nonstandard module stuff working fine, it's just that Google Base seems to want items with multiple values as a series of identical elements:

<g:label>Label 1</g:label>
<g:label>Label 2</g:label>
[download]

But XML:RSS doesn't seem to support outputting that type of XML, at least not that I can tell. For nonstandard namespace elements with a single value, it's working fine.