Hi @wob unfortunately this is not possible, c4dpy is a regular Cinema 4D executed as command line with the NoGui, and as all NoGui Cinema 4D execution, GeDialog is disabled.
So its a limitation, and as said by zipit there is no Tkinter shipped with Cinema 4D in order to force people to use Cinema 4D dialog.
So possible workarounds are:
Install pip and install any 3rd party GUI module (but this is not officially supported by Cinema 4D so you may have some odd issues, but it should work).
Execute a python script before from a system python installation that will create the Dialog and later execute c4dpy to do the Cinema 4D work.
Cheers,
Maxime.