Update: ok, so I found it wouldn't work on many games actually (I had only tested the ones I'm familiar with which are mostly Mattel). It won't work on Burgertime, Buzz Bombers, Atlantis (IMagic), Dreadnaught Factor (Activision), Blockade Runner (Interphase), Lady Bug (Coleco), Tower of Doom (INTV Corp), and I can assume many others.
From this information, it looks like it should only work with Mattel games that have that standard olive screen with text at the startup. It appears pausing interrupts some sort of graphics change from the title screen to the ones used in the actual game. I don't know much about how the exec ROM works, but if it doesn't work on an actual Intellivision, could it mean the ROM I'm using for emulation is a bad dump?
Any button the game allows you to start the game with should work (using a side button, if allowed, is probably the easiest). My method on a real Intellivision is to push a side button with my index finger, and exactly two frames after letting go, attempt to press both 1 and 9 with my thumbs. Alternatively, pressing 3 and 7 to pause the game should also work. Unpausing you can do with any button.