Frustratingly, no.
If you insert a photo into Messages and hit Markup, the first view gives you a bunch of pen/pencil/brush options, with fixed sizes per brush. There's a plus button that allows you to insert shapes and text…but you can't pinch-zoom to scale the text. You then have to tap around again to set the font size and color.
It's a weird, neglected-feeling interface. They could borrow a lot from Instagram's story editor.
What the author of the app can do however is creating an <x-Apple>OS photo markup plugin that provides the more usable mask ‘aid’ tools. A feature I haven’t seen utilised once since it’s inception.
Yeah, that's definitely what he should do. Very surprising he didn't do it already, this is the perfect use case for markup plugins.
Just tested it out and I was easily able to put an emoji over a face and resize/rotate it with pinch/drag type motions. Anything like that in the default Apple app?
Granted, there is nothing like the automation features in OP's app, so at least that part is interesting.