There was a recurrent requirement to 'practise' RHAG engagements. I did one at Leuchars. I cannot recall the speed restriction, but it was traditionally done on the 'approach end' cable, starting as far back as possible towards the start of the runway, and engaging both burners. I would guess less than 100kts would be right.
If I recall correctly, in an emergency the hook was 'normally' to be dropped AFTER passing the approach end cable, if rigged. Apart from anything else, it would make a mess of OC Ops' runway otherwise!