I'm trying to set up a manager order to automate weapon production if the available unequipped weapon stockpile falls below 10, but I can't seem to find a trait for 'unequipped' or 'in stockpile' or 'available to be equipped' ... something along those lines at least.
Does anyone know of a trait or other method I can use to set up this order?
You asked this quite a while ago, but I'll answer anyway. All conditions work on "available" items. Anything that is in the possession of a dwarf, being hauled by a dwarf, used in a building, forbidden or dumped is "unavailable". So generally, it just works if you set the level at 10.
The caveat is that, anytime someone hauls a weapon, it will become unavailable. You will notice this if you decide to send one of your weapons to the trade depot. You will instantly make another one. If you are putting weapons in bins, *all* of the items in the bin become unavailable when the bin is picked up. The bin is picked up when anyone takes a weapon out of it, or puts a weapon back into it. Because the bins can hold a lot of weapons, it means you may go under your threshold every time someone sticks their weapon in a bin after they stop training.
There are 2 things you can do about this. 1) Live with it. You will end up with excess weapons. Meh. 2) Don't put weapons in bins.
Especially be careful if you have a high number of items being built. So if you are building 10 maces in your work order, every time someone picks up a bin, you will make 10 more maces. It's quite easy to get up to 100 extra maces that way. I almost never set my work orders to more than 1. That way you only ever make one thing per work order per day. It seems slow, but that's like 300 maces a year. It's fine.