The implementation of this suggestion would be highly dependent on exactly how Modartt's algorithm works in terms of sound generation. With something like a leslie on a B4 emulator, the suggestion wouldn't pose any problems as there is a clear route in terms of sound generation:

Organ waveform -> Leslie (this part could be separate fx)

With Pianoteq, however, the route is not that simple and nothing about the way Pianoteq operates would make me think the initial sound is passed in a modular way onto a 'sound processor' which could be made a separate effect. It is my guess that all parts work together to create the sound. This of course means we can't just put an external sound 'into' Pianoteq.

I could be wrong but this is how I interpret the situation and I would imagine if this is such, the request won't be possible to implement.

It's not just that Pianoteq would then make a very interesting effects processor. It's also that we would gain even more realism, because we could mimick the reaction of the piano to events other than playing, say: someone slapping his hand on the lid, or hitting on a snare drum in the vicinity of the piano.

What says everybody else?

Then, i would like to have the possibility to have an audio input on the Pianoteq, with the possibility to make a preset with the sustain pedal always down (for not having to plug a MIDI input, a keyboard, a pedal only for that)

this would be for passing the sound of my instrument through the Pianoteq harmony strings effects ... and have a sound more like the D2 of Martenot.

Then, the VSTi should be placed as a VST fx, in an insert slot instead of a VSTi slot.

is it possible to do that ?