well you can't just use a 1k resistor because the afm in these cars are actually square wave generators. i don't know what happens if you don't connect the afm, but it certainly uses it for timing. i can't really think of anything right now, but if you will have a bit of time and money soon, i'd try to come up with something very temporary and get an ignition controller, see if you can score a haltech ig5 to accompany your f3, or preferably sell the f3 and go full standalone.
hm maybe you could somehow try to get a pulse to the ecu that varies with RPM as a bandaid solution. at idle karmann freq is what 18hz or so? so like 18hz at 750rpm engine speed? that's like 1100rpm at 750rpm. hm. if you could ground the ECU's AFM line for every time any coil was fired, you would get a 3:1 karmann freq to engine speed ratio which might be a reasonable estimate. you might want to actually have the karmann freq have that higher ratio to 'simulate' boost.
it really won't be a meaningful respresentation of flow on a boosted car but hey it might be a start.
i don't know much about electronics, but maybe you could have the Ks wire from the ECU to the AFM cut from the AFM, and hook it up to all three of the ignitor's coil outputs through a resistor or something. the ignitor will "fire" similarly to how the AFM 'fires.' the only thing that would be different is the pulse width, but it might not matter if the frequency is the same. don't hold me responsible if you try it... just an idea.
shiva