I've never tried this but it'll work. You could use a provisional cast on, work the intarsia flat then seam and finish it in the round. You'd then undo the provisional and work in the round down to the brim. Maybe add a stitch or two to the intarsia for the seam.
If it's a large section of intarsia, you might just knit the whole thing flat.
I'm knitting my first intarsia sweater in the round. It's worked by knitting short rows so it's like knitting the intarsia flat. Here's one way to do that: