how to refactor path duplication in .mrconfig section headers?

[Adam Spiers]

If I have multiple repository paths all similar but spread across
different .mrconfig files, e.g.

in ~/.mrconfig

    [.config/mr]
    checkout = ...

in ~/.config/mr/config.d/CLI:

    [$HOME/.git-repos/zsh]
    ...

    [$HOME/.git-repos/mutt]
    ...

and in ~/.config/mr/config.d/GUI:

    [$HOME/.git-repos/urxvt]
    ...

    [$HOME/.git-repos/fonts]
    ...

then the DRY fascist inside me is irked by the duplication.  I'd much
rather write this in ~/.mrconfig:

    [DEFAULT]
    lib = MR_GIT=$HOME/.git-repos

    [.config/mr]
    checkout = ...

and then ~/.config/mr/config.d/CLI would have:

    [$MR_GIT/zsh]
    ...

    [$MR_GIT/mutt]
    ...

and in ~/.config/mr/config.d/GUI:

    [$MR_GIT/urxvt]
    ...

    [$MR_GIT/fonts]
    ...

However this doesn't work - I'm guessing that variable expansion in
the section headers is done in Perl, so is not influenced by the
shell-code in the DEFAULT lib?

So is there any other way of avoiding this duplication?

Thanks,
Adam