I think that software vise there is not problem to have transports assigned to keys, it is just a matter of "define modes" for the keyboard. But it do require some button on keyboard, and maybe a double tap to tell the software "now you are in transport mode".
But if Cubase have it, well i have no idea, but i think the idea is sound. But probably if you have a controller keyboard you will use knobs and buttons to implement it rather then keyboard keys. But for an electric piano with just a button or two, one probably would do it the way you suggest using the keys.
And to Mike an UMX 490 so closei , but i actually do have at least one device with transport control a Tascam 1884, but also a Korg N364 and a Kurzweil K2000 and even a roland arranger RA-95. So lot of old hardware here, but i doubt the transport control is standard "maybe the Tascam controller board".
Right now it only send channel, program bank info, and handle some standard GM parameters via sliders like pan, vol, reverb, chorus sustain and modulation for my soundcanvas 7 in the main program.
But i will try to implement control surfaces as iframes for the gear i own.
If anyone want to make something for their own gear, i can share the little i know. Or help out with the million of bugs coming out of fast but sloppy, just send me a message.