See the package patch at 35147@debbugs.gnu.org.

To answer my original questions:

- This does not seem to enable writing onto HFS+ partitions.  A quick
  Internet search tells me that Linux should support HFS+ writes by
  default.  Not sure why it's not working for me.  Is Guix missing a
  kernel option?  We have

--8<---------------cut here---------------start------------->8---
CONFIG_HFS_FS=m
CONFIG_HFSPLUS_FS=m
--8<---------------cut here---------------end--------------->8---

Maybe something else is required.

- Indeed hfsprogs does not build easily on GNU/Linux.  So I've used one
  Fedora patch as done on Arch Linux.

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