> 
> However, we have toyed with other ideas. One of which is to introduce
> IUSE=prefix in prefix.eclass similar to the USE=multilib approach. I
> don't really like this idea because it exposes the use flag and we
> don't want it exposed to the users.
> 

If you don't want it exposed to users, then use a USE_EXPAND variable.
Like zmedico said in EAPI 3 a normal use flag could be hidden but
probably better to not have to migrate everything with prefix support to
EAPI 3. Out of the existing USE_EXPAND variables USERLAND makes most
sense to me but we could also introduce a new one too.

Regards,
Petteri