I have a mini "spy cam" but it's a pen-type. The lens looks out 90 degrees from the longitude of the pen. I can easily get it looking out through the body tube, but I want to see the downward view. Would a well placed mirror just outside the body tube work?
Yes, but you will find that using dedicated compact cameras will work better than adopting your pen-type camera to this role.
For example, this place offers convenient shrouds for Mobius, Runcam, 808-type cameras:
See what you rockets are up to in the air. Our camera shrouds are the safe way to mount your Mobius or 808 camera to your rocket.
www.additiveaerospace.com
Estes used to have a rocket kit that did this very thing. I wasn't sure if the mirror being close to the lens would affect focus.
If you are working with small diameter airframe Estes-type rockets, you may find it all but impossible to mount a camera inside the body and build a compartment to protect it from ejection charges. There is simply not enough space for that in most low-power rockets.
In that case, consider mounting something on the outside of the body.
A simple, cheap 808 camera attached via a Velcro strip works well, and can be moved across multiple rockets.
More info here:
https://www.rocketryforum.com/threads/mounting-keychain-camera.51867/