I think something like this would require an enhancement in the mod. It would basically take whatever you type and pass it as a parameter to a LUA script (which is what we do now anyway) but would require some hooks and binding to connect the keyboard command with a new type of chat window.
There are ways of creating custom menus and then you can simply use a few keyboard commands to play the sounds. Just like you do now with the standard game voices. One issue with that is, you cannot control who can play what. Everyone can play anything. Not really a problem, we just organize things by rank to have some incentive (how ever small) to gain promotion. Also, do we want every 0 level pubber to be able to come on and spam RAGE over and over? I don't.
The other problem, there would obviously be limits on the number of sounds we could have. Again, not a problem per-say and some would argue we have way too many to begin with.
So - for these reasons I've never seriously considered using the custom menu system.
I'm not sure of a way to create a custom prompt. There are also limits to the number of custom commands available (remember we used to be able to type !hax and it would play the sound?).
But........what seems like it could be a compromise -- given the limit of 63 custom commands (we're using maybe 10 of them at the moment), we could create other custom commands for the more frequently used sounds, !howyadoin, !bullshit, !hacker, !rage or whatever. That might make it a bit easier to execute the command without the overhead of typing "!play xxx". But this gives us the flexibility of being able to run all the commands using !play if we choose.