change temporarily to custom clef

[Orm Finnendahl]

Hi,

in contemporary music sometimes unusual "Clefs" are temporarily used
within a piece (for example for string instruments clefs where notes
indicate the contact point of the bow between fingerboard and bridge).

I guess for designing custom clefs one has to define a custom font but
I wonder whther there is another less complicated way by making a clef
temporarily transparent and putting some Postscript Markup in its place.

Can someone point me to relevant information in the docs or give a
short example (I can handle the Postscript, but don't exactly know how
to make the Clef temporarily transparent)?

--
Orm