This is a large part of what I was looking for! There are two more parts:
1) being able to re-generate multiple times after editing the shape of the land. This way I can get the shape I want and re-generate until the terrain/vegetation/... are what I want. Say a checkbox on the parameters pop-up after an edit, to say whether we might try again.
2) I'd like to be able to make maps in pieces which I can join up afterward. I could do it by editing the blueprint by hand to include the required edge and then filling in. It'd be nice to have an addition to the parameters popup to allow me to say, in effect, "copy the top row of the current map to the bottom of the map to be generated"
Wonderful tool! Thanks muchly!