The validation code is here https://github.com/NodeBB/NodeBB/blob/master/public/src/client/register.js. You can write a plugin to validate your own inputs.
Custom Maintenance Page
Is it possible I can make a custom Maintenance Page and use that one instead of the one comes with it?
If so, were would I make the changes?
I'm guessing this isn't possible?
You need to modify https://github.com/NodeBB/NodeBB/blob/master/src/views/503.tpl preferably in a custom theme.