MAX485 – Does It Need Decoupling Capacitors?

I believe the general rule is 1 cap per Vdd pin.

Are you using a ground plane? If so, don't bother attaching the caps to Vss pins. However, if you are using a ground bus instead, then yeah you should attach the cathode of the caps directly to Vss.

While not exactly what you're looking for, I have used power-management ICs to accomplish this. For instance, the TPS2113APW. I prefer this specific chip because it allows me to make dual-powered devices that can operate with either a wall-wart or off the USB, automatically preferring wall-power if it is available.

If you don't need dual-powered, you could use something like the MIC2545A

Ultimately, any capacitance "behind" the power-management IC (i.e. hooked up to the IC outputs) isn't "seen" by the USB; the bus only sees the capacitance "in front of" the IC (i.e. hooked up to IC inputs).

You still have to worry about inrush current - the "plus any capacitive effects visible through the regulator" part of the spec - but those ICs also have variable current limiting. Figure out the parallel resistances that you need to have 100 mA limitation and 500 mA limitation (and optionally n mA limitation if you want to limit wall-power), and then use FETs to short out the resistors as needed to enable various limitations.

Through these chips, I have attached PCBs with several hundreds of uF to the USB, and a DMM set to fast current max verified that the inrush during attachment did not exceed 100 mA.