Remounting tmpfs

[7e9wc56emjakcm]

On a system I'm porting to guix I have 2GB tmpfs with subdirectories
like /tmpfs/etc that I remount to /etc with an overlay filesystem.

The current way I do this in systemd is making a service dependency
between the /tmpfs and /etc mounts that mkdirs /tmpfs/etc and
/tmpfs/etc_work, but AFAICT filesystem definitions in guix can only have
filesystem dependencies.

Are there any other ways I can do this without copying/pasting/modifying
gobs of core guix code into my system definition?  Like somehow
appending (mkdir /tmpfs/etc) onto the tmpfs filesystem service start
procedure or something.