This is powerful stuff, but I can't help feeling like it's adding unnecessary bloat to an already-enormously complicated command. What is the advantage of:

    curl --variable "pwd@secret;[0-31]" \
        --expand-user daniel:{{pwd}} \
        https://example.com/

over, e.g.

    curl --user daniel:`command-to-fetch-password` \
        https://example.com/

? That command may be as simple as an existing one, like cut.