The coolant routing on my 300D is different than most of the ones I have seen in WVO conversions, and I'm trying to figure how to tap into it with a frybrid-style conversion. Does anyone have experience with this particular model?

The main difference is that when the coolant comes out of the engine, instead of going straight into the heater core, it goes to the aux pump. Because of this and of the fact that there is no monovalve on these models, I was thinking I could go in series rather than parallel. The coolant would go from the engine to the WVO system, then return to the aux pump and the rest would be the same.

No it won't work unless your heater is on.
If a 617 motor go to hard ware store a buy 2 brass tee's .mine was 3/4x3/4x5/8 for the hose I used.
You cut the hose after it comes out of the head going to heater,and clamp your tee in,install a short hose and put a ball vavle in(this way you can shut of heat when you can't run wvo.
On the passenger side you will see a pipe coming from heater then a rubber hose,cut hose install tee,attach your long hose to run around radiator an back to heat exchanger.