I'm curious to hear the answer to this too! I've never done a top down raglan, but am curious about them.
My knitter's instincts picture this... (please chime in, folks, if I'm wrong! Hopefully my brain isnt' playing tricks on me as I envision this....)
Seems like a provisional cast-on would be ideal, like the one I have on the cast-on page. You'd do this cast on, to get to the other side of the sleeve stitches you've put on hold. Then continue the body, and do the other side the same way, casting on to span the gap (the under-arm). From that point on, you'll only be working the body stitches, (which will include those cast-on stitches). When you're done, it's time to work the sleeves. There are the stitches that were on hold, and then the cast-on stitches in the under-arm area. With a provisional cast-on, you could just undo the cast-on to expose live stitches, and you'd be all set to knit the whole arm, from that point down.
Or, instead of a provisional cast-on, you could pick up stitches along a standard cast-on. Might be better in the end, since provisional cast-on's can be tricky, at least that one I have a video of can be. I often end up with a big loop of a first stitch, and I haven't figured out a solution yet! Another provisional cast-on, which might be better (no video yet, sorry), is to pick up stitches in the back loop of a crochetted chain. When you're ready to expose the stitches, just unravel the chain. That one should be quite reliable, I'd think!... But again, I haven't done this kind of sweater, so hopefully I'm not missing something.
Now I'm very curious.... what kind of cast-on does your pattern say? I may have to do this kind of sweater next, just to try out my provisional cast-on idea, if it's not the standard method.