Are there any guidelines to injecting power at the USB 3 device side instead of having it powered from the host? Basically making it self powered?
The easiest option would be to buy a powered USB hub. 24v is a common supply voltage for a larger hub, so you could simply plug the hub into your 24v source with no additional components at all.

Alternatively, you can disconnect the PC USB power lines and use your own 5v supply generated from a regulator. This is essentially what the powered hub will do.