I understand that it would be simpler to have the monochrome subs use the same layout as the colour ones, but would you consider using a more traditional layout instead? It seems to me the sort of people who would prefer to use the monochrome subs, would also prefer to have them presented in a more traditional, always centred way, like all the great Blu-ray labels do (Criterion, Synapse, Masters of Cinema, BFI, Arrow).
I hate nothing more than subtitles that are positioned below where a character is located in the frame when there are multiple speakers, as seen in the image from Update #14. I can hear the characters speaking and so can easily identify whose voice it is, even if I cannot understand the original language! When I watch a film with subtitles, I do my utmost to keep my focus AWAY from them, almost only looking at them in my peripheral vision, so I can focus on the actual images. This is a lot easier to achieve if the subtitles are less intrusive, less attention grabbing. The subtitles that feature on the music video from the Kickstarter page are basically perfect.
An alternate sub presentation is a great thing, but I don't think just a difference in colour is what is wanted most by a large section of pledgers.