Hey, 
I downloaded MAST a few hours ago and I love it. Here are some suggestions on some tools that can improve MAST :)

Drag 'n Paint
Being able to drag with an prefab selected and have it paint continuously as you drag the mouse. This would be great for floor tiles, Walls, and the like.

Flood Fill within selection
Drag out a selection on the grid. After you confirm your selection the tool will flood fill the selection. 

Hopefully these additional tools make sense in how I described them. Thank you for making this awesome tool! :)