OK, I looked it up in the parts fiche and I see the problem. You could leave away the guard guiding thing and attach the hose to the guard with a zip-tie or so, but then the guard would propably wobble around like a pendulum.
Your KTM dealer has already tried to get a genuine KTM spare part before ordering aftermarket stuff, I assume? If that isn't available anymore ask at someplace wehere they are building Harley choppers. I think these guys need extra long brake lines frequently when they install their bar risers and extra long forks. They should be able to tell you a supplier.
When you don't find anything in the US (which I can not really believe) there is a company in Germany where you can order custom hydralic lines to your specs. When you want they send you a quarter mile of brake line, so that you can brake down your bike in the garage while watching TV in the living room. I had them make a rear brake line for my rear-brake-to-left-bar-end convertion after installing the Rekluse clutch. That one was 1.70 metres long. Website is
www.spiegler.de Download the brake line catalogue and look up which fittings you need.