usage/help: Print switch value placeholders

In the Guix Workflow Language I'm accepting a switch --input with a potentially complex value. The switch could be --input=NAME (for a file name NAME) or it could be a mapping --input=NAME=FILE to use a file with name FILE instead of the expected NAME.

Previously, I explained this in the help text like this:

     "  -i, --input=NAME=FILE  Specify workflow input NAME, optionally mapped to FILE"

With Guile Config, however, it looks like this:

  --input  -i  Specify workflow input NAME, optionally mapped to FILE

Is there a way to print placeholder values?