I've seen it mentioned in passing a couple of times that "all headgear is considered 'light'," but in-game, helmets are clearly labeled as "medium" or "heavy" to match the armor they accompany. So my question is, will wearing a "medium" helmet negate the bonuses from the "Light Touch" perk?

Thx,
MT

Until the Lonesome Road all headgear was classified as light and could be used to repair one another if you had jury rigging. Be it combat helmets or power armor helmets. The Lonesome road introduced heavy headgear.
Now to answer your question.
Light Touch uses the same reference ID list as Travel Light. Which mean so long as that perk is functioning so is Light Touch.

While wearing the elite riot helmet and light armor I suffered no loss of speed so travel light was functioning normally which means Light Touch was also working.

So anything pre-LR should work. Cool, thanks for the info.

As stated above, Light Touch works off of Travel Light's coattails. Travel Light (and hence Light Touch) uses a form list as a reference to determine what it applies to. In this case, it's a list of armors that Travel Light doesn't work with. It's called travellightbadarmor list or some such. I guess there's more armors and clothing it applies to than doesn't so it's easier to make a list of those that don't. Since this form list is in the main game file, everything in dlcs can reference it and work properly. Each dlc uses a special script to add things to these perk lists like add guns to the cowboy list, add ammo to ammo lists etc. Lonesome road's special script adds all the new medium and heavy armors to the bad armors list, but none of the headgear is added. That means none of the headgear, even those that say "heavy" are going to effect Light Touch. No problems.

-Gunny

You didn't read the last line of my post did you?

I read it fine. It just didn't particularly matter to me because all i wanted to use was Salt-Upon-Wounds' Helmet. Thanks anyway.

If you read the lase line of my post you'd have known that all headgear will work fine not just pre-Lonesome Road.

...aaand i look at Salt-Upon-Wounds Helmet in-game and it says... Light. As does my Combat Helmet Reinforced, both of which i could swear said Medium yesterday. So it appears that creating this topic was, uh, totally pointless. How embarrassing. I'm blaming the painkillers. Thanks anyway for the detailed explanations, should be useful for future reference and for others who are confused.