Sean, I've never had or worked on an AC6, but I doubt there's much difference between it & the one below (a spare parts AC7a).
I assume the brass knob you mentioned is the bearing for the governor, as in the forefront of the picture below.
To be honest, if it were mine, I would bite the bullet & pull most of the motor down. That way you can replace any rubber grommets that isolate the motor from the bedplate, and give the parts inside the housing a good clean & lube. I wouldn't worry about removing the block with the electrics that drive the governor, but I would take the top plate off the housing so you can remove & clean the governor bearing & take out the spindle to clean it & the hole in the base where it sits. I dont normally worry about removing the little disc on the bottom of the housing that the spindle bearing sits in, but clean it out using a Q-tip & thinners, & repeat until a q-tip comes out clean.
When I reassemble these, I use car motor oil in the spindle & governor bearings, as I've found lighter oil (singer) I use on wind-up motors seem to disappear after a few months of use. If you've got a heavier oil meant for electric motors though, I'd use it
Then I just use a bit of vasoline on the governor & fibre gear, and you should be good to go.
Once the housing cover is back on, if there's a felt gasket around the top of the spindle hole, I'll load that up with oil as well, before attaching the motor back on the bedplate.
When I've opened some of these 7a's & later motors, the inside of the housing has been full of dust (while others are spotless), so with the dusty ones, I've used the vacumm cleaner down where the governor enters the electrics block to try and suck out any dust that might be lurking in there, but I'm sure you'd do a more thorough job by removing the endplate bolts from the block, and pulling the block & plate away from the housing to get better access, but I'm never keen on moving the wires if I can help it. It just creates more work if the insulation crumbles or a wire snaps, and some can be a major PITA to replace.
I've only got one Garrard that still has the type of noise you describe, and it's the first one I did, but I only used singer oil on the bearings & gears. I haven't ever gotten around to redoing that one because I hardly ever use it, but the others I've done since, with heavier oil & vaso, any noises have never returned.
As you suspected, there's very little to these motors, so I think it would be well worth pulling down & lubing properly, otherwise every time you play more than 2 or 3 records, that irrating noise will sneak back & have you pulling your hair out
PS: None of these early Garrards & Collaro's etc that I've owned have had removable/replaceable brushes, so I doubt yours will either. They just seem to keep going forever unless one of the magnet coil windings burns out.