First of all, Arca Swiss is no longer a Swiss based company. Second, the Arca Swiss swallow tail - as far as the common one (marketed by Arca Swiss, France under "Classic") is concerned - is no longer patent protected. Their clamps are and a newer swallow tail plate (Monoball Fix) as well.
Otherwise Arca could stop machining expensive ballheads and clamps and make a living by sueing the patent thieves
I actually don't see a good reason for Fuji not to offer Arca-type plates. Tamron does include them with their 150-600 G2 and 70-200 G2 and it's great to have not another part to buy and fiddle with. For Sigma Tele-lenses in Sports version, there's a TS-81 available. Pretty expensive at around 200.- € but improves the balance of their 150-600 Sports on a Gimbal a lot. Just putting a longer plate underneath? Why not, it just feels sharper in my hand when carrying the heavy lens. I prefer the TS-81 any time.
There's a bit a downside, unfortuantely: There are a couple of clamps on the market which don't fix the plate by tightening a screw but using a "real" quick-release technique. A lever like that one from Arca
Or a click-module like that from FLM
I'm nearly sure RRS has something similar. The Arca version can be found as a cheap (and cheaply designed) copied version from China. I tried it and I'm not recommending it. But eben the very expensive Arca clamp has a weak spot which is abolt made out of ordinary steel without any surfae protection, starts to rust soon.
Especially the FLM version is in terms of handling very similar to Manfrotto's 410 PL and 200 PL: You put the camera into the clamp - click - fixed and cannot longer escape the clamp. One hand is enough for that. Downside with the FLM system: It's not 100% compatible to real Arca plates, the dimension of the swallowtail is highly critical. Most plates don't fit into the FLM clamps without a bit of milling
The Tamron foot would need some work to fit in, I hesitate to do so. But I don't doubt Tamron has made it according to Arca specs - and the Arca lever clamp can be adjusted, contrary to FLM.
On the other side, Fuji also sells grips with Arca-swallowtail (not many camera manufacturers do so...) Like milandro said, doing the Arca swallowtail would not cost much more than the normal foot of the 100-400. Which I find ridiculously small, by the way. I'm alwas a fan of "if you can do it well at nearly the same price, do it well".
