Odie,
The old 'Speedo Cable' trick is barely used now, and there is a specific RR MAGERD kit for the job.
It is basically a baffled pipe that is vented to Atmos which screws onto the Hot Shot ignitor in place of the fuel pipe, and the engine is then run at Max Dry for a few minutes. This reverse purges the ignitor, and is a pretty effective way of cleaning the ignitor. The trouble with the speedo cable method, is that it used to eventually knock the tip off the ignitor, with the associated FOD problems!!
As you quite rightly say, in the event of a non-light up, the Working Line Limiter will then cancel the nozzles to Normal Dry Range, once the Nozzle position (Aj) has reached 70 - 75% with no pressure increase in the jetpipe.
Cheers,
Flipflopman