I'm now giving a try to your (widely demanded) Multiple SCMs, which appears to be promising.
The configured SCMs are Subversion (3 URLs: data source, release branch 1, release branch 2) and Mercurial (1 URL: mainline).
The main problem I'm facing now is that no change is reported to the build page, preventing us to see who has committed what when something gets wrong (or, for the record, much better in a recent case I had).
Any idea?