I'd like to use the Ruckers Harpsichord and make use of the registers with my triple pedal unit. At the moment I can ctrl+click and assign the register to the pedals but I have to hold down the pedal for it to work. Is it possible to temporarily turn the pedal into a kind of on and off switch? If so how?

I have assigned the sustain and sostenuto pedals to 2x8' and Upper 8" respectively and the una corda allocated to the buff stop as configured by Pianoteq.  Depressing the sustain or sostenuto switches the respective registers on, with the other automatically going off.  The una corda works and needs continual depression, as configured.

I have yet to go through the rather lengthy process of having several configurations of the Ruckers so that, for instance, sustain provides U8+4' and sostenuto L8.  Looking at my settings, controller 64 is set to MyPresets/H Ruckers  II Harpsichord L8' + U8'.  You need to create fxps with all the instrument configurations you want, then set the midi mapping accordingly.  For me, switching from 2X8' to U8 is a standard instant switch on a double manual harpsichord without having to push or pull handstops or push/pull the coupler.