All you need for a lock is a heat source.
I understand that a Russian fighter has a rear firing missile.
Aerodynamics will be a factor although in a slow moving airframe, such as a Nimrod, probably insignificant.
In a fighter firing forward the missile may be given an extra 400-500 kts of energy. In a Nimrod firing backwards it would have a negative energy of -240 kts.
Against a 707 the issue is one of geometry as much as forward or backwards. The 707 can outrun the Nimrod at high level. A turn away outside the engagement envelope would cause the missile to abort.