Just learned something new while watching the videos of Will Hooker's college permaculture class. Many plants have extrafloral nectaries (ie, not flowers) which exude nectar. Scientists theorize the plants do this as a symbiotic relationship with beneficial insects such as ants and predatory wasps, who keep other herbivorous insects in check.
So for permaculture design, which emphasizes scattering beneficial insectary plants through your garden or food forest, this greatly enlarges the number of species you can use to achieve this goal. I suspect that if you have a wide diversity of plants in your system anyway, then you might not even have to deliberately plan any insectaries for nectar. You'd still need to plan out the pollen-providing plants, though. Of course this depends on more information on when exactly the extrafloral nectaries produce; I don't know whether the plants exude their nectar all the time they're in leaf, or just in certain seasons, or only if they're suffering from predation, or what. I'll have to start observing plants to figure this out!
Extrafloral nectary plants by family
Extrafloral nectary plants by genus