>> We don't have to use the `#:key' syntax if we don't want to force that Chez adds a mode where `#:key' is read in our sense as long as there are letters in the alphabet.  `#&key' looks equally fine.  Apart from that, if Chez is the only system using `#:key' for something else, we can simply ask its author.  It seems that he has already once abandoned the `#:key' syntax for uninterned symbols: http://www.r6rs.org/r6rs-editors/2004-January/000043.html.
>
> Gambit uses #:foo for uninterned symbols.

Gauche does too:

(keyword? :foo) => #t
(keyword? ':foo) => #t
(keyword? '#:foo) => #f