You cannot have a Nyquist effect plug-in that generates at a point - but it can almost be done if you create a very small selection to replace with. This lets you use the CTRL + R “Repeat last effect” shortcut to repeatedly generate silence (CTRL + R currently cannot be used to repeat Generate and Analyse effects). As waxcylinder said (if your Audacity clipboard is free) you can instead copy the silence then insert it repeatedly at the cursor point with the Edit > Paste shortcut (CTRL + V).
I think it also serves as a simple demonstration of how the output of an effect (process) plug-in need not be tied to the length of the selection. The code for this plug-in is essentially just one line:
(stretch-abs gap (force-srate *sound-srate* (s-rest 1)))
where “gap” is the variable that determines the length of silence.
How it works is that (s-rest 1) creates silence of length “1” (which would normally be the length of the selection).
(force-srate sound-srate …) forces the silence (which is a sound) to the same sample rate as the track,
and (stretch-abs var … ) then stretches it to the value of var (in seconds)