Running a command before source package build

I'd like to build a package off a git repository without a control file, or more specifically, generate the control file before the source package is generated.

In sbuild terms, I'd like to run a script such as `cd %b; debian/rules control` via --chroot-setup-commands for the source package (only). Running the command in the chroot is important for the environment (and the only sane thing to do anyway, for security reasons.) The actual building the generated source package won't need any special hooks.

Is something like this supported at all?

Alternatives exist, but I'm not keen on using those. Essentially, I could manually make sure that the control file is always up-to-date on a different branch and use that for building on Launchpad, but that means a lot of additional overhead (since every push to a build branch would need a merge to the launchpad build branch as well).