The direction should be tagged as railway:signal:direction=nominal/reverse regarding to the OSM way.
This is very easy: it will not happen. This is neither a decision of you or me, this is just about 10 years too late to be of any chance. The existing terms "forward" and "backward" are used in OSM about everywhere for "in/against direction of OSM way". (Old style) public transport relations? Check. Tagging how many lanes an in which direction a given piece of highway has? Check. You get the idea. Inventing something new for railway tagging is just an invitation for another flamewar[1], so we will just stick with it.
Eike
1) look at Michaels mail about his mechanical edit to introduce DE-ESO: prefixes for German signal names/states on talk-de@openstreetmap.org