Use other flows under standard billing to do iterative planning, spec building, and resource loading for a substantive change set. EG, something 5k+ loc, 10+ file.

Then throw that spec document as your single prompt to the copilot per-request-billed agent. Include in the prompt a caveat that We are being billed per user request. Try to go as far as possible given the prompt. If you encounter difficult underspecified decision points, as far as possible, implement multiple options and indicate in the completion document where selections must be made by the user. Implement specified test structures, and run against your implementation until full passing.

Most of my major chunks of code are written this way, and I never manage to use up the 100 available prompts.