I have a really awful habit of posting problems mere minutes before I find the solution :P
Turns out sysd finds when USB is connected by polling /sys/devices. By doing a mount -o bind over /sys/devices, I harmlessly disable its polling without breaking anything important. And I can just umount the binding when I'm done
Have I mentioned that I am absolutely in love with the hackability of the DR?
I'll post a runnable program of it for other would-be devs to have fun with, once I have it a bit more cemented.