How do I specify bags in the management condition interface?

Solution 1:

You could try removing the condition for empty bags and instead have the stockpile set to [g]ive resources (and perhaps take as well, I am unsure if the empty bags produced are considered "produced" by the furnace) to that glass furnace, and ensure that it can contain 50 or so sand bags.

This will require that you also ensure it has fuel, but since you mentioned magma I am presuming you intend to make a magma glass furnace so that should not pose an issue.

This way, you will always have 50 bags either empty or full in that stockpile but they will only ever be used by that glass furnace for collecting sand or making glass, so you are free to put other bags elsewhere and they will not be used.

Do note that eventually your glassmakers will become very fast at crafting and will likely outpace your sand collection unless you have many dwarves on it.