Actually I think there was a Litecoin implementation too.
Which is almost identical to bitcoin ..
the trezor can basically use any satoshi blockchain wich marginal modifications
As long as the crypto works the same everything is possible (though not with TITAN). There are very few actual problems we've encountered.
So far we've just played around with it. once 1.0 is out our plan is to start working on it more seriously.
Maybe I can help with that.. I already have good knowledge about the blockchain transaction format (https://github.com/xeroc/python-bitsharestools/blob/master/bitsharestools/transactions.py
What made me halt on developing trezor for bts is (besides of lack of time) .. the implementation of the wire format on the USB interface.
I think the best approach would be to define new protobuf messages for bitshares transactions .. but I haven't read into the trezor implementation (trezor-emu -- python) .. to much to can come up with a 'good' AND flexible solution to enable bts signing on trezor ..
But I totally agree that trezor will certainly not work nicely with TITAN .. but it would work with BIP38 and 'deterministically' derived keys in the tree .. (TITAN is not directly 'deterministic' if you don't know the 'shared secret' from the stealth address) ..