Right now, we can only place block signals along straight lines. But maybe it should be possible to put them along perfectly diagonal tracks, where the track is laid at a 45 degree angle and doesn't divert anywhere.

Will not happen, sorry. Signals work by searching for entities in Axis-Aligned boxes. A diagonal would require either a massive search box that would overlap any other tracks nearby (rendering it useless), or it would require a multitude of smaller search boxes (greatly impacting performance).