My only comp idea was to make it someting easy to organise, and that means no scoring and nobody having to sign things off.
So how about:
You use something like the NSRA published short or long range averages as a handicap, to even things out (or equivalent)
Then you nominate a future event, NSRA meet, open English match, Grand Prix whatever and then you take your average for that event (only one event and once nominated you can't change it!).
Then the winner would be the best of (comp average)/(declared average), or you could use the Macrae thingy if that helps (whatever that is, I don't actually know!)
Fun bit is you obvioulsy have to pick an event where you think you will shoot above your average, since you declare in advance you might get lucky with the weather or stuffed by it. It should mean you can do this if you shoot 3P, air, prone, whatever.
Nobody has to run it, you just log your average and where you got if from (in case anyone is suspicious - surely not) and then nominate and event you know you will shoot and then report the result. (one of the resident IT bods will likely be able knock up some web/forum based spreadsheet thingy that you can enter the numbers in)
Someone just needs to define a start/end date for the period of shooting, (one indoor and/or one outdoor or just one whatever?)
Please help refine/edit/reject its just my ramblings after breifly thinking it over at the w/e since I did suggest a comp I thought I'd better come up with something.....