I didn't see any discussion.
I think this is a good idea. There are cases where a protocol element,
or an API expects a URI, and within a single logical application made
by a single team of developers there is a need to pass a fairly
application specific identifier across this interface.
For this use, currently people invent their own scheme names from
anywhere in the scheme name space, encouraging them to do so from the
set "x-*" will avoid clashes with non-private use schemes being