Has anyone done this?
In a word and a letter? F-no! lol
Be easier to find and purchase a 4000. Could probably even retrofit a 4000 mobo in a 3000 case much easier than you'll find someone willing to make an AGA adapter. I say screw AGA though. You need a really fast processor (at least 40mhz 030) if you want to use the higher res desktop screens with more colors with any sense of ease. What makes more sense is investing in an RTG card for those circumstances.
And the few games that take advantage of AGA, which you can still hardly see a difference between that and ECS (not talking Demo's, stills or a pinball game or two)... meh. Not worth going cuckoo for cocoa puffs over IMO.
ECS or RTG I say

Unless of course, an inexpensive AGA solution for our legacy machines was available... LOL!
And $1725 for the machine with such a lovely ePay description? Wow. Hope they're happy with their purchase I guess.