One workaround for the Gulp/Grunt/DFSG issue would be to build a Debian package and host it on the koha-community repo. I would be willing to do this.
Unfortunately, all of the frontend build tools seem to either:
a) have license issues preventing them from having an official Debian package (Grunt, Gulp),
b) not be popular enough to have a package or be viable (broccoli, others) or
c) have limitations in functionality that make them not worth the effort (Jake).
I agree with Owen both that this is strongly needed and that it would be doing complex enough tasks to make maintaining a separate, limited build script for packaging/tarball installs untenable.