I don't think registering an alias in this situation is a good idea.
There will be folks who have [log in to unmask] who don't have
[log in to unmask] accounts.  There are several sources of
conflict and consternation waiting to happen.

I've seen it here (a long time ago).

Maybe having customer (or list-manager) issue an appropriate CHANGE
command (or some site-administered RYO-script) would be better?

YMMV

/Pete