After a teeny bit of reading, from what I can understand, Karplus-Strong is actually a form of waveguide synthesis - waveguide synthesis is just different arrangements of delay lines and low-pass filters and, for the sake of simulating sympathetic resonance of other strings in instruments such as the guitar, feeding the signal into other delay lines.
For example an additional delay line might be arranged differently to simulate the bridge or pickup of an instrument...
With that in mind, a Waveguide module might be possible if you adapted the visual scripting system to just include audio routing, delay, and low-pass filters. Of course the filters and delays would need to be automatable. That's how I'd do it, since it seems the arrangements differ depending on the instrument you are trying to imitate. Again probably not trivial.
I'm not entirely sure if the filters are always low-pass, either, but I don't know much about it. It probably is, since it's always used to simulate the reflections throughout the instrument and such. That's my thoughts on the waveguide part.
On another note it also seems the Phase FX is not polyphonic - it'd be useless for instruments that can play more than one note at a time.