Ok! For anyone keeping score, I figured out how to get my custom script working! I found and modified a PHP script instead, and the #NextCloud Snap was able to access and run PHP on the script!
The trick was finding a place that the snap's `nextcloud.occ` command could access the shell script, then making finding a place that the sandboxed NextCloud snap itself could access, and finally making sure the scripts were owned by `www-data` and in the `www-data` group!
So I created the command shell script in `/var/snap/nextcloud/common/commands/roll.sh` and pointed the shell script that gets run by the sandboxed NextCloud snap to where I have my data storage accessible via `:removable-storage` in `/media/nextcloud/scripts/evaldiceroll.php`!!!
I HAVE DEFEATED THE SNAP!!!!
I'll write a post about this eventually, but I actually have some time sensitive things I /really/ need to finish first...