Hi,

I haven't looked closely at your patch, but here is the rule for
copy-build-system if I'm not mistaken: the files matching #:include
preserve the path relative to the first argument.

For instance with this rule

("/source/aaa/" "/target/bbb/" #:include ("foo"))

the file

    /source/aaa/parent/foo

is copied to

    /target/bbb/parent/foo

(I haven't tested.)

Does that help?

-- 
Pierre Neidhardt
https://ambrevar.xyz/