Hi Steve,
I might burst your developer bubble, but it’s actually simpler than that.
You can top up Monzo via bank transfer using details provided by @simonb (click arrow in right corner to see with nicer formatting):
You setup standing order from your bank and done. Bank transfers SHOULD be automated by now if 9-digit user code is provided, so it should be quite painless process to automate top-ups, if you plan to top-up by same amount regularly.