Hmm, although I obviously don't know their implementation, I would hazard to guess that it might not take any extra database space at all. There's already a distinction between an "equipted" item and a non-equipted item. This could be another state of that field, {equipted, locked}. I also don't think one more field is really that bad for space. Consider the following worst case (which may be simplified).

Assuming they use an entire 32-bit word for the new field:
1,000,000 customers. Say each has 3 characters and each has 200 inventory slots
That is 1,000,000*3*200 slots ~= 2.25GB of data. That really isn't that bad for a nice functionality (in my simplified view of the world), and that field could be re-used later (possibly).