Don't do this. Install a transfer switch. A simple plug and another power source feeds power back into the grid. It's absolutely against code. If you remember to flip the furnace breaker off before you do this, then it's not going to backfeed, but this isn't something you should rely on remembering to do. It doesn't cost much more to do it right.

Sounds not-up-to-code and potentially dangerous

Makes sense. My mind went into a less safe direction, I’m glad I asked before following it.