use of crows foot or chens notation to draw an extended ER diagram
It seems to me that you should be able to draw your ERD in the same way that you would for a regular relational database. Take a look at the wiki article for Object-Relational databases; there is a lovely example image on the page that might be of use.

While the functionality of an object-relational database might be somewhat different than that of a regular relational database (joins are understood etc.), it seems that they can both be represented in the same way.

It should appear that it is not so much the design of the database that is different, but instead the functionality of the underlying database management system. Crow's foot notation is currently the recommended standard.