Cool idea, but it conflicts with fluff. While the Inquisition is the only entity outside of the titan legions to control Imperial titans, they only have one legion (to the best of my knowledge), and it's makeup is already defined. Speaking of which, I would love to see someone make a Legio Ordo Sinistar in 40K scale.
In addition to titan fluff we have to look at the fluff for the SoB. They generally get discarded/outdated warmachines. Now if you came up with a backstory about finding some STC, then maybe. Of course they would have to get a forgeworld to build these for them. Since SMs, IG, titan legions, and Skitari would all be of higher priority, it still wouldn't make sense for the SoB to get a titan.