Populate custom fields on Add New Page?

Still working on my database of composers and music and now on plan D!

My latest crackpot(!) idea is to set up a page per composer (easy) then once that page is saved, present a button to create a new page for an item of music for that composer (on both admin screen, and on the page itself for permitted users) similar to the standard Edit New button: wp-admin/post-new.php?post_type=page

However in the ideal world I want to the link to be something like:
post-new.php?post_type=page&category=song&composer_id=current_page_id

Then a new page is created with Category pre-filled as “song” from the argument and the composer_id custom field populated with the calling page_id. If the custom field can be made subsequently uneditable that would be even better but there maybe another plugin can do that for me based on permissions.

I can use Custom Field Template plugin to get pretty close, but I want to be able control the content of the composer_id field so we maintain some degree of data integrity.

Any thoughts? Simpler to modify the Custom Field Template code?