Under which JAR? JAR-FCL? Doesn't really apply here. JAR-OPS? Replaced by EU-OPS. However both allow manual landings with less than 300m RVR under certain circumstances. For example all those HUD equipped CAT IIIa approved airplanes with correctly trained crews may land with less than 300m RVR as normal operation.
However the question was a somewhat non-normal case, if a go-around below DH is mandatory for CAT IIIa operation. And it seems the answers are somewhat split on that. Some operators do allow it (even under EU-OPS rules), others do not.
Personally i would be very interested how those operators that have aircraft with both autoland capability and the ability to do manual CAT IIIa approaches and landings (for example 737s with HUD) cover that special case.