Tie the line to the jib boom between sr2 & sr3,using a clove hitch, tuck the free end under sr2 silicone tube and make sure that sr2 & sr3 are pushed tightly together to trap the clove hitch and pull on the free end of the line to take up the slack. Treat the other end as recommended, with a loop. I have used that method on all 3 rigs for 2 years and never had it slip yet, even in a gale on a C rig. If you want to adjust the height for any reason, just separate sr2 & sr3 a bit, then adjust the clove hitch.
As far as I can see, it is entirely legal as it uses a knot to fasten the line to the boom.
David.