In short, kde-build-metadata/dependency-data supports specifying branch dependencies. Although kdesrc-build will parse this data, it doesn't obey it at this point.
It should do so. The format should be already described in the kde-build-metadata README.
Git commit a456c51b7abd6cf7af1ac037b4ef78cf1c511d87 by Michael Pyne.
Committed on 30/12/2012 at 02:46.
Pushed by mpyne into branch 'master'.
kde-projects: Support "catch-all" dependencies.
The most noticeable effect is probably that it should be damn near
impossible not to have automoc build first now, at least in comparison
to other kde-projects modules. It also should take care of the vast
majority of the code needed for bug 312324. Still needed is handling for
M +57 -6 modules/ksb/DependencyResolver.pm