Is there some documentation which indicates the schema for newly created custom objects? Specifically, I'm interested in the field name which represents the last modified time of a custom object. At least from the Analytics tab it looks like at least for custom record types, it's labeled 'lastmodified' rather than 'lastmodifieddate' (as it is for many other NS objects). I'm guessing it's the same for custom lists but it would be great to confirm somewhere formally.

By custom object, do you mean custom record, maybe? Does the Records Catalog not show it?