how to configure default sign key for particular user?

[Robert J. Hansen]

> Otherwise, any application [which knows only username/email] has
> to be know also the specific keyid to override gpg's default
> selection (which I'm guessing is the first key in the keyring);
> this seems wrong...

It is wrong.  You should file a bug with the software package which is
mistakenly using UIDs as unique identifiers, as they are not.

> it should be configurable in gpg

Probably not.  The bug is in the software package you're using, not
GnuPG.  Adding new features to a package to remedy the brokenness of
another package is usually counterproductive.