It is possible to start it on-demand via the command-line. That is how krdp is developed and tested. You will need to pre-authorize krdp to the portal system, though.
If you can shell in, I don't see why not? It's a command-line thing. It's intended to also be usable for things like Ansible. If it doesn't work, please file bug reports!
That's documented here: https://develop.kde.org/docs/administration/portal-permissio...