Quadcopter and rocket engines...
If there isn't a YouTube video of this, I'm going to be sorely disappointed.
Don't worry. If there isn't now, there will be when I'm done.
a few of the pins are outputs to devices (including an array of rocket engine igniters, don't ask lol) which would trigger if I toggled them high.
You don't toggle the pin HIGH. You set the pin to INPUT and toggle the pull-rail HIGH and LOW. The rail can only source/sink 50 microamps (0.05 mA) because of the resistors. Do any of the outputs connect to high impedance inputs? That might cause a problem if the input doesn't have a pull-down or pull-up. For example if you are driving a Darlington transistor pair to switch current to a motor, relay, or ignitor the 50 microamps might be enough to cause significant current flow.
If the pin reads as HIGH when you set the pull-rail to HIGH and LOW when you set the pull-rail to LOW then it not connected to anything with an impedance lower than about 100K.
Ahh right, I get you now! Sorry, I was a little slow on the uptake. All the stuff which is high impendence stuff is controlled via relay, so it's fine on that front.
Much the same sentiment as above - if you have rocket igniters connected to pins on the micro, you really need pull-downs - of about 1K - on them anyway.
Arguably, you should have each of them passing through a NOR gate, the second input of which is (in common) driven by a separate output pin through an inverter, requiring both a high and low output from the micro simultaneously to actuate the igniters.
Noted, and I'm adding that in now anyway as it's a sensible safety precaution, on top of all the mechanical interlocks :)
Thanks all of you for helping! I'm going with what johnwasser recommended though, simple and effective!