the standard talks a lot about graphical user interfaces. i'd be
interested to learn how it's been implemented.
We're using an variant of what's in the draft (an earlier version) in our web
client. The details are rather complex, but basically what happens is that the
client uses the alternate representation in order to avoid having to parse and
otherwise deal with Sieve syntax.
do any existing desktop GUIs use this mapping?
I don't know.
do any existing web GUIs use this mapping?
Yes, see above.