SSO: Allow customization of 'return' keyword

We are using a CAS server for SSO which expects the return URL to be contained in a 'service' key. Would it be possible to make the keyword customizable for SSO? So we could get this:

https://auth.example.com/?service=https://example.userecho.com

Instead of

https://auth.example.com/?return=https://example.userecho.com