Thanks for addressing this use case. Your proposal, however, runs counter to any standardization effort: Long live OAuth!

A better approach would be to switch from a password grant to the use of authorization codes (instead of passwords) to obtain the access token. This is fully within the OAuth framework and does not introduce fusionauth-specific hacks into the solution.

We have created as simple html page that redirects to the fusionauth authorize endpoint with grant_type=authorization_code. The browser handles MFA as usual. Upon redirecting to this page, the page can harvest the authorization code for the user to copy. From there proceed with into authorization code in place of a password.

PS: Long live OAuth!