Hello,
this is actually just two questions and potentially a request.
Are single body part flags available in json yet?
Also, is it intended to eventually split up “pairs” of things like gloves and shoes into single parts?
Actually this is totally possible in the code, it’s just that it wasn’t handled well enough to be mainlined when it was presented (but all the behind the scenes stuff was and went in just fine).
To make an individual item you can either use the [ARM], [HAND], [LEG], or [FOOT] cover tag, which will automatically make them spawn in left and right variants. (They will usually spawn in pairs, but occasionally spawn in singles). When crafting you will only craft one, and it will allow you to select either the left or right side. If something has multiple sided cover flags (such as [ARM][FOOT]) then the game will put them all on the same side (so that would cover either the right arm and left foot or the left arm and left foot).
If you wish to have something specifically be on a certain side You just stick either a “L_” or a “R_” in front of it (i.e. [L_FOOT]). For an example at this look at the special glove that the Thriller drops, it’s worn on only one hand.
As for what the eventual goal is, I’d like to split up gloves and whatnot into left and right sides, the difficult part comes in getting it so that it doesn’t force more micromanagement and work onto the player to maintain and manage all of their equipment.
When Kevin outlined the do’s and don’t’s of collectible items, he talked about having items that auto stack to become new items. Maybe when you have both R and L gloves, they become a single “Pair of Gloves”. Disassembly could either prompt splitting into the R and L equivalents, or cutting into rags.
I am not sure how interactions with single items from the stack would go. What would happen when you reinforced the pair? Would you have to split it first? Would the code be written to try to reinforce everything in the stack? What about when they get damaged, can only one be damaged in a pair?
Which is exactly why I haven’t done it yet. 
I’m still working on answers for a few of those questions, which I need to find before I can resubmit the idea.
Ah, good news, then!
EDIT: Testing confirmed, works as promised!