Good idea, Mike! I just ordered the same mixer from Sweetwater yesterday so I can do the same thing! Been using a Fender portable amp (the little package they market for that sort of thing), but now I can carry the mixer instead, and plug my wireless headset into that, and my guitar into the Spider IV 75, and not have to mess with that other amp. :D
(Later...) decided not to use the headphone-to-mp3 in - I'm just running the guitar into Input 1 (with the High Z button depressed), the headset into Input 2, and the left out into the (guitar) input of the Spider IV. Works like a charm.
As an aside, my Spider IV sits under my computer desk, as that's where I normally play. My computer headphone jack is connected to the mp3 in on the Spider (so I can play along with iTunes or videos on the 'puter), and my JTV-59 into the guitar input on the Spider. I'm really lovin' it!