Yes, this "can" work, but it is not ideal due to the many unknown variables on someone's host PC. Hence it is not a default option, and we would not recommend it. Issues can range from USB VBUS being out of spec (as it is, it is allowed +/- 10% which puts it in on the edge of our needs), and most "correctly designed" PCs will not provide more than 100mA unless a device is enumerated that asks for more. I'm not sure if all version of PX4 do that correctly. 100mA is insufficient for most use cases.
In a strictly "QGC config" mode, I can see this work, but it is not something we plan on supporting on this version board due to those risks above. This also opens the door for someone to leave that power feed in-place while then later connecting the power module which will create a serious power rail shorting problem between the module and the PC.
However, we do have a design already figured out that can take USB power for QGC/Config type of applications, and it validates the VBUS through a smart fuse/mux before using it. This will allows us to globally claim support, and if someone's system is not correct or out of range, we protect the HW.