Feature Request: Support logic or shell execution to control values in .gitconfig

I sync all my dot files (including .gitconfig) among several machines
and it's currently not possible to put conditional logic in many
fields (any that aren't considered strings to be executed as shell
commands ie aliases, editor, etc).

My specific use case is the email address. Normally I want my email
address to read:
mcclurem@$HOSTNAME  [off-list ref]

I use this to track which machine I'm committing from etc.

I propose using something reminiscent of bash syntax, either:
value = $(SOMETEXTTOEXECUTE)

or

value = `SOMETEXTTOEXECUTE`

Is this a feature others could get behind?

-Morgan McClure