Can this sp2 lone pair resonate?
Whenever an atom possesses both a π bond and a lone pair, they will not both participate in resonance


The sentence means that only one, either the  bond or the lone pair will participate in resonance if the atom has both.



For example, in a pyridine, the nitrogen has a lone pair and it is also attached to a  bond.

Here, however, only the  bond participates in resonance because it is in the plane perpendicular to the molecule, while the lone pair is not.





Methyl isocyanate is best represented by a group of three resonance structures:



Here is the 3D structure of the most significant resonance structure of methyl isocyanate: