A standard hex wrench - I think it was 8mm, is what is needed to install the arms. If you want to remove the self-extracting caps a set of needle-nose pliers can remove them. Just put the tip of pliers and hold the handles open to wedge the tips in the caps. A slight turn will loosen the caps without a huge amount of pressure. The caps are left hand threaded - so Righty Loosey. I have removed and reinstalled the cap on one arm and had no issues with damaging the finish.

Yep, I know the cranks themselves attach using a hex wrench, it was the caps I was talking about. I don't think I have any needle-nose pliers fine enough to fit in there, hence the pin wrench. When I was fiddling around with loosely putting them together when they first arrived the drive side threads got damaged (I think there was some swarf in the threads of the spindle) and I had to remove the cap on that side to get the bolt out so that I could clean it up. I was worried that a few thousand km down the track I'll have a similar issue when I take them off to service my BB etc., the caps will have managed to clog themselves up with dirt or cold weld themselves to the spindle or similar, and I might need more force than pliers or my pin wrench can exert to get them out.

Thus if it was a standard fitting for some other more popular brand of crank I figured I'd just buy the right tool off AliExpress for a few dollars and be done with it. But if it is a proprietary thing obviously that won't be possible. Does anyone else recognise the spline pattern?