Yes, that’s what makepanda actually does when cross-compiling: it expects an existing copy to be in the PATH.
That does make building a lot less automatic, though; and sometimes, there’s a particular change to interrogate that follows a change to the source base. If we wanted to do this, then we should probably implement makepanda to build a second version of Panda just for interrogate, which would be rather complicated. Failing that, the buildbot should just build two trees of Panda automatically, one with just interrogate built for the host platform.
Anyway, I may get around to that if Steve doesn’t get around to setting up a 64-bits buildbot before then.