Hi -
The guys at FliteTest posted a video recently where they made an ultralight RC plane using a Rogallo wing made out of Christmas wrapping paper (). It used 1/16 hobby plywood for the structure - I made a CAD model (Solidworks) out of the design and printed it, If you are interested the files are posted on Thingaverse (https://www.thingiverse.com/thing:4716526).
I have an old toy microquad (HAK903) that I would like to repurpose for the plane, but I am not sure if the gyro control will allow me to run it in a vertical position.
I was wondering if anyone has ever tried something like this?

Without the ability to change the configuration of a closed source FC like the one used on the HAK903, I don't think you're going to be able to use that. You're probably better off just getting a cheap / basic FC that supports Betaflight and/or INAV.