Is it ever possible for an continental plate to subduct under an oceanic plate?
No it is not possible because the continental crust's density is lower.
Conversely, oceanic plates can subduce under continental ones because they are heavier. This is because continental crusts are formed by granites and sedimentary materials. Their density rounds $2,7g/cm^3$, granites ones, while oceanic crust rounds $3g/cm^3$, basalts and gabbros ones.
Oceanic plates start to subduce at a certain age. Presently there are not pre-Jurassic oceanic plates that do it. This is related with the stack of hidrated sediments over the igneous materials.

An exception to this are ophiolites, where oceanic plate obduce continental one. This is not happening presently at any place on Earth, but there are fossilized examples as Semail Ophiolite at Oman.