Probably the best approach to this is to allow dropdowns/radiobuttons to get their values directly from the form definition, instead of from an enumeration. So you could have an input that looks like:

{{{input|country|input type=dropdown|values=Angola,Bolivia,Canada}}}

Then, once wiki-parsing of the form could be done before SF's own parsing (that's a separate "bug" on here), you could also have calls that looked like:

{{{input|country|input type=dropdown|values=<ask format=list>[[Category:Countries]]</ask>}}}

...or even use more complicated queries.

eep wrote:

And, more preferably, not just limited to Semantic MediaWiki's <ask> queries but also DPL's <dpl> queries...

Any ETA on when this might occur?

Sure, DPL too, or anything else that can generate a list of values.

Sadly, there's no ETA on any of these.

This can be done as of version 1.4, using the "values from category" parameter for field declarations.