I do understand that the configure service uses the deployment mechanism to apply the configuration and therefore can not work when signed artifacts are activated.
A nice way to overcome this problem would be a field where one can drop a signed json config that gets then sent to the server.
It can work if you make and sign the artifact yourself. IIRC the build line goes something like this:
mender-artifact write module-image \
-T mender-configure \
-t raspberrypi4 \
-n my-artifact \
-k <SIGNING_KEY> \
Not quite as convenient as what your proposed, but possible.