You can change the max dirt in the prefs
He means he wants you to
start with 1000 dirt, not
be able to carry 1000 dirt.
On-Topic: I may be able to make a little script that starts the player with said amount of dirt (as determined by RTB prefs). Then all you'd need to do is build the map without using dirt blocks.
I am terribly sorry if I am misunderstanding what you meant in the original topic.
But I'd just use the TW mod and build your map out of normal blocks, but still give players a shovel and dirt placer. I'll make you a script that starts people with dirt if you want.