You're quite right that working-out first-chance averages in domestic cricket is not possible to do as accurately as in Test-matches.
For me, Flight-Path schemes (whatever you call them, "Hawke-Eye" seems the most popular) are largely unnecessary. Most of the time you can tell by slo-mos whether the ball would have hit the stumps. I certainly don't believe it's faultless (in fact, it seems to me to have a consistent problem with thinking full, leg-side balls are further towards the off than they are) so the messing around taking 12 cameras around county circuits isn't a neccesary precaution.
Catch-carrying can be told 99.9% of the time by the catcher. Certainly it can almost invariably be told better by players and centre-Umpires than any camera, because they are closer to the incident most of the time. Generally, you don't need cameras and for me they shouldn't be used to judge on catch-carrying.
So, in summary, dropped catches, missed stumpings and the like can always be ascertained without cameras, snickos only rarely prove Umpiring errors, most of the time they prove them correct, lbws are helpful with camera-angles and red-zones but most of the time a bad lbw will be guessed by the players and spectators.
In summary, you can get a fairly accurate first-chance average for any cricket. I dug-up one for all my team-mates last season. Made interesting reading.