Thanks for the reply Chris!
I checked that thread out and tried it on my work computer. It would *sort* of work, in that you can assign a keypress shortcut to bring up an on-screen volume control, and then you could use up/down keypresses to raise/lower volume, but then you'd need to 'return' to close the volume popup window. Certainly a way to accomplish controlling volume, but not very elegant.
Currently I do have my vol up/down remote buttons mapped to the Plex volume shortcuts, but the Plex volume shortcuts are not as 'absolute' as regular volume controls (e.g. you cannot make the volume any louder than it is currently set, you can only make it softer.
It looks like there are some third party windows applicaitons which let you make your own macros/keyboard shortcuts, that's where I'll look next. I was hoping for something out-of-the-box, but I'll see what I can find.