I am no auto electrician, so could not say for sure as to wether your plan would work or not, I would ask a few questions though.
I have a 58 with original wiring, The semaphores do not flash, the light stays on all the time the semaphore is out.(constant supply = semaphote stays up, light stays on) So the 'flasher relay is not realy a flasher unit, rather a switch.
Your car was fitted with the Sweedish 'ears' that I guess flash like regular indicators so your car probably has a 'flasher relay'
If you wire directly from the relay you may find your semaphores 'flapping' as well as flashing.
Stock......the new custom