ok-- some bad news. no, there isnt’ protection on the USB for overcurrent. the 5v regulator is underpowered. TT was designed without the fantastic-future-vision that a grid would be attached, let alone anything needing over a 100ma (ie, a keyboard).
further-- i don’t think any firmware hacking is going to fix it-- as i’m not sure the the uC has much to do with the current measurements (despite the libavr headers) in disabling the USB power switch.
what this has reminded me, is that i need to open-source the eagle files. (first i need to clean them up so they’re useful). and second, the next revision of TT need this fixed (along with having a few more i2c ports on the back).
a drop-in hack/fix would be to replace the 5v reg (U5, right on the bottom board edge) which is a SI8050— but i’m not sure there is a higher current drop-in replacement.