My mountain bike ('04 Gary Fisher Wahoo) doesn't see many mountains, I'd like to make it faster without having to spin like a maniac. I already have a touring triple crankset (Suntour XCM) that probably won't be seeing any other use, is a swap feasible?

Probably,but chainstay clearance for the bigger rings and the possible need for a different FD also for the bigger rings are things to consider. The crank could also require a different spindle interface and or length.

If you already have a 11T rear cog, your big ring already provides you with at least 100 gear inches.....which is plenty enough for most folks.
If you want to simply get rid of your very low gears, I would simply swap out the cassette for a road cassette with closer ratio cogs..... like an 11-23 cassette for instance.

I'm using a more touring oriented set of rings on the Fat Tired Rider, which is a mountain bike frame. 46/36/24, front derailleur is an XT.

At a minimum you'll have to fidget with the front derailleur, probably reposition it. From what I can gather with a quick check, looks like the XCM takes a 126mm BB. Most MTB these days are using 110/113, maybe a 118. So you'd probably have to swap it as well.

Might be easier to change out the rings on the existing crank.

thanks for the advice, I'll look at swapping just the rings first
i'll probably just leave it alone though, i've got some other projects on the plate already, this was more of a passing fancy