How to export downloaded files for a system profile?

[wifi]

I'm trying to create an archive containing only the store items that were 
downloaded to build machine A's active system profile, bootstrapped from source 
with no substitutes. Machine B could be an air gapped system, machine A in the 
future needing a reinstall without an internet connection, etc. Machine B would 
be running a minimal guix installation to import that archive to.

Ideally, this archive would not contain any binaries produced by machine A to 
save space when machine B already contains the necessary tools to consume the 
archive and bootstrap it.

I've been digging around looking for a way to build a complete build dependency 
and runtime dependency graph of every package and their derivations, but there 
doesn't appear to be a straightforward way to do it. If such a complete graph 
could be built, it seems to be only a matter of finding the leaf nodes and 
exporting those from the store.

Am I on the right track?