For example, if my one-handed skill is at 90, and I use an enchanted ring to boost that skill to 105, then go bashing away at Falmer for a while, will the skill level bar continue to fill, or will it stay at 90 until I unequip the ring?
No, it doesn't stop accumulating xp for that skill. I don't see a reason why there should be a penalty for wearing a fortify one-handed ring? Maybe it accumlate a bit slower xp? Most likely the changes in time is more important then the absolute skill lvl. A fortify one-handed just boost the damage not the xp.