No, these aren't perfect substitutions, but:
1) If you time your key press, you can shoot rockets in UT just as fast the instant rockets in UT2004. It just takes a little practice to get the timing down (it's NOT just a matter of hitting the button as quickly as possible -- you have to time your press with the loading animation).
2) If you want to switch between regular firing rockets and instant firing rockets in UT, you can either:
Bind one key to:
setinstantrocket 1 | onrelease setinstantrocket 0
Or bind a first key to
setinstantrocket 1
And a second key to:
setinstantrocket 0
I have my mouse thumb button bound to the onrelease bind; this basically gives me the best of both worlds, which I can't do in UT2004.
Some set commands seem to be blocked by various anti-cheat protections, but I've never had this one blocked.