I need the equivalent for hook_preprocess_node for custom pages, which aren't nodes.
The particular page I need to theme is provided by the entity registration module 'node/10/register'. I can add theme suggestions for the page using hook_preprocess_page - but how to I add a theme suggestion for the actual content, so I don't have to rewrite the page.tpl.php?
Alternatively if there is an existing pattern to name the file then that would be even better.