Finding out if panelizer displays are in use
This info is not saved in the theme registry, and it's not really available per page or per node. However, it is stored per configured Panels display. Many don't have a title but since I'm using Panelizer, they do have a Panels display ID. Here's a query that returns those:

select did, title from panels_display where layout = 'mytemplate';


Entity IDs correspond to DIDs in the panelizer_entity table. Once you have queried for the DID you can query for the entity ID using the DID, like so:

select entity_id from panelizer_entity where did = 1234;


Where you replace '1234' with the DID you got from the previous query.

I have to do that second one for each DID that the first one returns in order to find all entities using mytemplate.