ERC-721 can be mimicked on Steem as of now. May not be the full spec as such but things like 1. uniqueness, immutability, cryptographic proofs etc. (2) Further, the ERC-721 as such doesn't create value. ie, there will not be any value stored in the token as such. ie, an ERC-721 token cannot store say, 1 Etherium in it. In Steem, using the escrow and vesting features, its actually possible to store a value to the token. Also instead of the token, it will be a regular account tied to a specific user using multi-sig :-)

We did a prototype for a game called PixelFarm and our drgaons and pets were represented this way.