I want to use a terrain paint as the "floor" for the outdoors area of some lots. But when I
look at the lot in CAW or edit in game, the terrain is only visible if the lot is in high detail mode. Will that be different when the world is actually being played? I don't want the terrain defaulting to grass whenever a lot isn't 100% loaded. It looks ridiculous.

You need to run EIG and perhaps open the lot too, to make game realize there is new paint. Or if lot is currently empty and placed in grid, just replace it and the new terrain paint will stay.