It appears I am able to use sling objects out of the box from sling:defineObjects in CQ just fine with Groovy.
However, is it also possible to have the benefit of cq:defineObjects and other cq taglib functionality in Groovy in CQ, currently?
This is not currently a feature. It would be possible to create a BindingsValuesProvider (https://cwiki.apache.org/SLING/adding-new-scripting-variables.html) which added these scripting variables. This could be done either as a product addition or as part of a custom implementation. If this is something you'd like to see added to the product, I suggest you file a DayCare issue with this feature request.
thanks for the info, justin!