You may be right, I will admit that I can't remember what was in the original NAVHARS of the FRS1, but the expanded acquisition modes were definitely in the FRS2/FA2.
But doing CAP without a radar (your own, a ground-based one or an AWACS) would mean relying on purely visual threat identification. The scene of action was over the horizon from the carriers' radars, so the only option would have been to have an FRS1 "stand off" and try to use its radar to direct the GR3s into visual acquisistion. Playing fighter-controller is a heck of a workload for a single-seat fast jet driver!
The whole SHAR A-A doctrine was based on using the radars of two aircraft travelling in opposite directions to minimise gaps in radar coverage (sharky's famous "coordinated pairs" tactic), and you can't use a GR3 in the same way. Obviously you can do SOMETHING, but how effective that would have been against radar-equipped missile-armed fighters is another story!
I could be wrong on this. I was a SHAR and then Harrier II man, but only ever as an engineer - never as a driver.
PDR