Does anyone have suggestions (or experience) for writing a justification to attend Pathfinder school as a 68W?

The most convincing argument would be that you need the knowledge for HLZ site selection for medevacs, and it will allow you to tailor your 9-lines to aircraft capacity and function. If you are planning to fill a future assignment on a small team, you could also state that it would be cross-training that will prepare you to be a productive member of a small team when you are not conducting medic tasks. It will give you a purpose when there are not patients to take care of. As others have said, there is really no great reason, but those examples would be what a higher authority might possibly buy. Best of luck to you on snagging a waiver and a slot, it's a great course and a cool badge.

Did you try calling the school? Dont listen to the folks saying medics don’t need it. Get after it!

If it's not in FMSWeb it likely isn't a thing. It also isn't one for the Pathfinder school either for a 68W. Obviously you can apply for a MOS waiver, but it's not a likely necessity. You can "what if" anything to throw in HLZ operations but the big thing is that the medic shouldn't be doing the HLZ ops, they should be working the patient, while the 11Bs/19Ds do the HLZ ops. If it's on the receiving end of the patient the HLZ would already be set up by a qualified person.
11B, 11C, (11Z must be assigned to a Pathfinder Billet), 15Q, 19D, 88M, 92R (skill levels 3 and 4 only), and 92Y (skill levels 1 through 3).

It's a pretty badge...everyone wants it.