I'm trying to have a unique building that the civ can have in multiple cities, but can only have one city producing it on any given turn.

So while City A is building a forge, then no other cities in the civ can be building a forge. But when City A is done City Y could then start building a forge.

Can this be done in XML?

<iExtraPlayerInstances> could be used for it... You should try. Maybe it's useful only when <iMaxPlayerInstances> is full, maybe not...

I think that to check whethere it's being built elsewhere before allowing it is definately a python task.
There's just no tag in xml (that I've ever seen) that can do that.
So, you'll almost certainly have to edit python.

Actually, you do and do not need python for this task. One of the globaldefines is the number of allowable instances of each building per city. Which is 1, by default. Final Frontier changes this to 8, but uses a python callback function to limit the number based on planets. So you could set the number higher, and then use python to limit buildings not on a list to one per.