OK, I consulted ktorrent upstream on this, and it seems it gets the
magnet protocol handling via /usr/share/kde4/services/magnet.protocol
which is a (imo) deprecated way of doing it (vs. the setting
MimeTypes=x-scheme-handler/magnet way).
A secondary issue specific to our current ktorrent builds is that we
still build it with kio_magnet support, which upstream currently does
not recommend.
So, I'll see if I can sort out the mess, and come up with something that
works better.
-- rex