Eric Meinert, from Canada, has just launched a Kickstarter project to fund his POST MOUNT disc brake fixture. Suitable for both frame and fork as with previous fixtures, and suitable for both 140mm and 160mm rotors. The fixture can be ordered by pledge on Kickstarter: https://www.kickstarter.com/projects/4608648/bicycle-post-mount-disc-brake-jig. It will subsequently available from his Incepi website.
This fixture now complements his other available fixtures which are available from his INCEPI website at: http://incepi.myshopify.com
There is an ISO disc brake tab fixture, below, which I purchased through a previous Kickstarter launch.
Additionally there is now a FLAT MOUNT fixture which I understand is likely to become the accepted standard for road bikes. I think the price for the fixtures is very reasonable despite having to import them from Canada: effects of Brexit not withstanding!
I have no experience of flat mount fitting on steel frames but The only commercial supplier I could find of the flat mounts themselves was Paragon Machineworks in the USA. However when I was at Bespoke 2016, Bear Bikes in Leighton Buzzard did have some on display and although they are not advertised as such I am sure they can supply them.