I was looking into this project as well.
First thing, is it possible to branch the Pak128 SVN folder?
I'd love we could work on a branch (which could be of course fully compiled) until Kieron's patch is committed into trunk, then we could just merge back Pak128 branch into trunk.
Roads will be the easiest to do. All of them will have both slopes (except probably 110 and 130 km/h, treated like railways).
I want to limit railroads to the gentle slope, so I plan to keep hidden tracks (i.e. without icon) in the pakset with both half and full slope (for backwards compatibility), while new official tracks will have ONLY half slopes. This could apply also to motorways and trams, maybe also trolley electrification. Same would apply for canals and navigable rivers, whereas streams would have both slopes.
Power lines and Schwebebahn will have both roads, as well as yet-to-be-painted narrow gauge tracks.