[Developers] Limiting prefix searches

[Alf Eaton]

I'm using Freebase Suggest to autocomplete music artists, albums and
track names, and would like to be able to limit album and track title
searches by the previously selected artist.

For example, this is currently possible:
http://www.freebase.com/api/service/search?callback=done&type=/music/artist&prefix=nirvan

I'd like to be able to take the guid of an artist from that set of
results and use it to limit a subsequent album query, which currently
(without the limits) looks like this:
http://www.freebase.com/api/service/search?callback=done&type=/music/album&prefix=nevermi

I see there's an mql_filter parameter mentioned in the response, but
can't find any documentation...

Is it possible to add further limits to a prefix search, or maybe even
to perform a prefix search using mqlread?