Got it, thank you for the quick answer ^^
After further research, I found that this will not be possible under the current way Unity works. When you click to place an object, Unity attempts to use the same click to select an object - usually the one you just placed. To solve this issue, I had to make MAST eat the click event before Unity could use it.
Simply put, MAST has no choice but to remove normal left-click functions in the scene view during placement mode, or you will experience an extremely annoying side-effect with each click. Hope this helps.