It is easily possible to register sling-model-exporters to component-nodes if they have a proper sling:resourceType set but would it make sense to have this option on the page level too? Any dispatcher or security related issues maybe?
If I understand you correctly, you would just start registering customer exporters as selectors selectors to the cq/Page resourcetype.
Technically I don't see a problem with that, but just be aware, that I can execute this exporter on pages where it shouldn't work. That means you need to write some code to ensure that this exporter is just executed on the right "type" of page, and not on any page. If you register that SME directly to the correct resource type, Sling does that for you.