Salvia Nemorosa, perhaps the most common salvia in gardens across the states, has been discussed about the possible opioid activity the several terpenoids found in the plant may or may not have. The link is of an article that shows that chemicals in the plant certainly have effects on the opioid system.  http://www.ams.ac.ir/AIM/0032/hosseinzadeh0032.html This article does not mean that the chemicals in the plant are automatically opioids. I think there is a chance that they are, but I think it is more likely they are enkephalinase inhibitors.
I think S. Nemorosa has some potential as a source of relaxation, sedation, and euphoria. It obviously has no where near the potential that opium does, but I think this is an interesting plant and we may want to do more research on it.
Their are other plants that contain enkephalinase inhibitors in them such as any in  the Lactuca species and Cichorium intybus. I think there maybe more. I am interested in finding these plants. 
I would like to think a combination of these various plants could be more effective as each compound is going to act a little different and also be metabolized at different rates, etc.

Does anyone know of other plants that may possibly have enkephalinase inhibitors in them? I suspect Salvia Splendens might but I have found no confirming evidence.

Adding two PDF's that are related, mostly only mentions the properties of Lactuca Virosa and its analgesic properties.