On a public_transport=platform, polygon, there is the walking line, mostly this is the tactile_paving line, so tactile_paving=yes, can highway=platform be used for this?
Just like the other highway= like path, footway, pedestrian, etc, for routing.
Routing over areas seems to be poorly handled at present, but my view is that you should not create artificial walking lines over areas.
Marking as tactile paving should be done to indicate the existence of tactile paving. It should be up to judgement of the routing software designer to decide if and when to interpret that as being the best line for a sighted person to follow.