Maybe you could have multiple personalities embedded into the Rx? although you then need a screen on the Rx to set them up, and that = $/£/€/etc
I disagree. Given that the physical interface for the various forms of TTL is different, choosing the right personality could be simply a case of selecting which connector to use.
A real world example: A usb keyboard that comes with a PS/2 adapter[1]. The protocol is completely different so obviously the adapter cannot be simply mechanical. The solution? The adaptor shorts a connection which inside the keyboard selects which interface chip to use. This can be a simple as wire direct to chip select on both devices with a not gate on one.
Suggested model. Have a port dedicated to TTL control. This gives us (probably) 8 pins to play with. I've not seen a TTL system with more than 5 which would leave 3 for a BCD switching mechanism. This allows for 8 different TTL protocols to be supported by one device simultaneously.
Canon / Nikon / Minolta (Sony) / Sigma (Do they have their own one?)
It's just a case then of offering different connections from the device. These could be nicely made by using the existing TTL extensions systems from the manufacturers in question... for those that want them.