I would guess that an emergency replacement tile needn't fit perfectly. The goal is to survive one reentry, not to meet a goal of X number of tiles surviving Y number of reentries. All that's needed is good enough.

I've read somewhere about emergency tile replacement kits, and as I recollect, they might have an assortment of tiles that would fit roughly within several shape ranges--not 31,000 unique shapes, but, say, 100 approximations. The gaps would be filled with some type of heat-resistant putty, sufficient for one reentry.

Storing the repair kit on ISS has an advantage over the Shuttle--there's room to install a small computer-controlled milling machine or table saw (something on the scale of one of those little Dremel table-top workshops) that's preprogrammed with every one of those 31,000 unique shapes. So, instead of the weight and bulk of a kit of several hundred generic tiles, you'd have a smaller stock of blank material and the machine. Identify by number the tile that needs replacing, and tell the machine to cut it.

The idea of a tiny machine shop aboard ISS makes sense, I think. It's certainly consistent with the goal of eventual self-sufficiency. Wouldn't it be beneficial if simple items like internal bulkheads and supports and work surfaces could be made on-site instead of waiting for a future supply mission? How about other emergency repair parts? As long as we're not talking about something that would affect structural integrity or safety, why not let the crew build what they need?