The way I'd consider that would be with a new item in the points list, for incline=*, which shows a new custom icon to be drawn up in the typ-file; that icon could be the bogstandard "black wedge" GB road sign.
There's not much resolution in 16x16 for more detail.
It would be nice to do OS-style chevrons, but icons don't draw relative to the map lines under them: they're all always the right way up. An informative sign plonked on the point is a decent best effort, I think.
I'd love to get it at low zoom levels, too, so you could plan routes sensibly, but I've never had a ton of joy with zoom levels. Any ideas, osm_nerds?