Eager loading EntryElementType from custom ElementType in a plugin
The article on StraightUpCraft should be able to point you in the right direction. PHP stuff is towards the bottom of the article.

Examples of Eager Loading Elements in Twig and PHP

Also this Craft hangout is worth watching, it gives a lot of detail on Eager loading. It's over an hour long though, so set yourself some time aside to watch it.

Eager loading Elements