Yes, there's an awesomely detailed teardown here, but it doesn't go inside the motor.
I've found part of the answer anyway : there are two torx screws hidden under the brushes (take the brushes out). This removed the white plastic bearing mount and allows limited access inside the fan housing - enough to trickle some oil into the bearing, and it seems a lot happier now.
At that point, taking the fan nut off might have allowed further stripdown but it looks as though it's got threadlock on and I'm not sure how to grip the armature to undo it or replace it : I'm too well brought-up to use waterpump pliers on the toothed pulley. But there may be some possible access if I need to have another go.
Replacing the bearings would be great, but they don't seem too bad with a bit of new oil. I think the problem would be taking that toothed pulley off : not sure how that is fixed. Maybe with a hammer.
At least it doesn't smell of burning any more