scramblins: Sorry to necro this thread, but it's still one of the top results when searching for this problem on google, so I figured I might help a few people by posting here.
I came up with a workaround that involves using autohotkey to press the "]" and "[" keys multiple times to effectively pause and unpause the game by pressing the spacebar.
It's not a true pause, but rather a hotkey that toggles between the minimum and maximum simulation speeds. However, it has worked better for me than mashing the "P" key over and over again while building.
Here's a tutorial explaining how to set it up:
www [dot] youtube [dot] com/watch?v=X7BfaCLyvsw
This requires installing an additional program called AutoHotkey which is one of many programs allowing to emulate a sequence of keystrokes when you press one key (Spacebar in this case but can be programmed to anything else).