Okay, I think I have it figured out. Took a little patience and a bit of penetrating oil to be sure I wasn't forcing anything. I watched a bunch of video tutorials on more recent DT Swiss hubs and while this older model Bontrager is a bit different, it still got me close enough.
It appears the drive side cartridge bearing needs to be replaced. I've watched an Asian video that shows the tech overhauling the original cartridge bearing rather than replacing it, but I'm not sure I'll go that route, unless it turns out to be accessible without punching out the entire cartridge. If I can peel off the seal and flush out the bearings and they seem okay, maybe. The cartridges seem to be pretty standard, should be easy to replace if necessary.
The freehub cartridges seem fine, but the pawls are really grungy and just cleaning them might wind up contaminating the bearings. We'll see.
One video tutorial seemed to imply that it's possible to use the axle as a punch to drive out the cartridges. Not sure I'd want to try that even if it's possible. I'm sure I still have some older brass punches somewhere around here.
The biggest expense may be the bearing press. This is my only wheelset with cartridge bearings, not really sure I want to invest in a press unless I plan to get other hubs with sealed cartridge bearings. They do seem good when everything is rolling smoothly.
I'll post photos later. They're on my phone, which is recharging. Might be helpful to someone else later with the same or similar hubs. These seemed common to the early-mid 2000s Treks with Race Lite Aero wheelsets.
I also found an obscure page on Trek's website with a few spare parts for these older hubs. It wasn't readily accessible via Trek's own site, but after juggling multiple Google searches it finally ferreted out a relevant page. Seems to be a quirk of Google -- if you enable cookies, scripts, tracking, etc., it "learns" and eventually bubbles up stuff that I've searched for unsuccessfully before, but finally unearths older pages that were there all along.
A couple of photos after a quick cleaning...
This photo shows the drive side cartridge bearing, minus seal, that's the only bearing that's grindy. Flushing with degreaser didn't help. The other non-drive side bearing and bearings in the freehub itself seemed okay. I might replace them all if I can figure out a cost effective way to do it.
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