This is rather exciting. Do you think that this method can be adapted to
build the pluggable transports bundles?

Yes.

I originally started working on this so I could submit a patch to include

the tor-fw-helper in the TBB.

But since the helper is only needed in the PT bundles,

reworking their build process seems the more productive route.

I'll see what I can do.

This is currently done
semi-manually, with a makefile that builds the pluggable transports,
unzips the bundle, copies in some files, and re-zips the bundle.

https://gitweb.torproject.org/pluggable-transports/bundle.git/blob/HEAD:/Makefile

We also have instructions for setting up a build VM from scratch--which
it sounds like is what Vagrant does--but I guess we would instead use
whatever is the build machine for the vanilla bundle.

https://gitweb.torproject.org/pluggable-transports/bundle.git/blob/HEAD:/Makefile

It would solve a lot of problems for us to have the PT bundles built at
the same place and time as the vanilla bundles.