You can reach whatever you can see. You can place a building anywhere in the viewport. You can open any chest that's visible to you.
Doesn't work for mining.
NEW: Configuration allows you to change the default reach value. The default is set so you can reach the maximally zoomed out viewport, and not much farther. You can increase this to a larger number to allow you to edit remote bases from the map view.
UPDATE: This mod is now maintained again! It will work in 0.16.