Rather than use the stencil buffer to mask out the region outside the reflector, if the mirror polygon has five or fewer edges (four is a common number), OpenGL's clip planes can be configured to clip the reflected geometry to the region directly behind the mirror polygon.
For each edge, calculate the plane that is formed by that edge and the eyepoint. Configure this plane as a clip plane (without applying the reflection transformation). Finally, configure one plane to clip reflected polygons to the plane of the reflector, as in the stencil technique.
This may be a viable alternative if hardware does not support a stencil buffer.