If so, how does one get into the insane game?
Ag flying is not a game, it is a profession. It is not an entry level job in any sense of the term. It is far from insane, but rather a calculated program involving trained, experienced professionals who have a background in all flight related theory, as well as entimology, chemistry, crop science, aircraft maintenance technology, and training in the particulars of low level precision application of chemicals to enhance crop growth, fight fire, protect or clear wildland, etc.
If not, are there opportunities for hours builders to go abroad and earn a crust doing ag flying?
No.
Don't build hours. Build experience. You can write all the hours you like in your logbook, and they are meaningless. Experience is priceless. Don't go in search of hours. If that's what you're after, save yourself some aggravation. Buy a good pen, spend a quite evening in front of a fireplace, and falsify it. it's just ink.
Experience, on the other hand, is hard earned, can't be bought, and can't be falsified or made up.
Two people can fly the same airplane for one hour, doing the same maneuvers, under the same conditions...one builds an hour of experience, the other logs an hour of time. The difference? You. Which one will you be?
Getting into ag flying isn't easy. Going abroad and hoping to get into ag flying is nearly impossible.
If you want to fly ag, it's certainly an attainable goal, but not an easy one. You're going to want a good aircraft maintenance background, preferably a mechanic certificate, at least a thousand hours of conventional gear (tailwheel) time in ag type or lager airplanes (not cubs or RV-4's), and these days, some good turbine time to get insured...and generally at least a thousand hours of ag time before anyone will hire you anyway. That's the catch 22, and it's hard to get hired without it. The question you need to ask is how badly you want to go this route?
As far as "building hours" doing ag...the number of hours you'll fly won't make it worth your while. Some years yes, some years no...but if you go after ag flying to fill or pad your logbook, you'll likely be sorely disappointed years down the line.
Personally, I've found little flying that's more fulfilling and enjoyable...but that's an entirely different reason to do it than building hours, and the reason that most do it is to put bread on the table. More than a game, it's a living...a hard working, hard earned living.