I don't think it's tricky at all, if you are referring to strictly software-based DSP....which would all be completed prior to the data stream reaching the sound card/audio device, regardless of the interface. Your use of the term "plug-in" seems to imply that you are referring to effects that are handled entirely in software by the player app itself.
However, you threw in the term "driver", too.....does that mean you use your sound card configuration applet to enable some DSP effects that are handled by hardware on the card itself? If so, then there is some question, though I would guess that the S/PDIF output from the card would be from the same data stream that is routed to the internal DAC chip.
No, S/PDIF and USB are not handled the same. A direct S/PDIF output must derived via an internal sound card, while USB is (more or less) just data output. There are several different methods of handling audio streams via USB....."Windows USB Audio Devices" conform to a standard that allows them to be used "driverless", but most are limited to a maximum of 16 bit/48 kHz operation (the Benchmark DAC1 being a notable exception.)
USB audio interfaces that are gaining popularity as sources here, but originally designed to allow notebook-based recording at higher bit depth/rates, generally require their own drivers to handle data transfer to/from the external box. The data transfer can be handled in various modes, each having advantages/disadvantages.