I take it that by "inconspicuous" you mean that your tools will be safe from would be thieves when you leave your bike locked, right? So the appeal would be to people who use multiple bags and packs and don't want to forget their tools or worry about them. I think if you play up those features you might sell a good amount of these. At first look i got the impression it was targeted to the "weight weenie" crowd, as they don't care to have bags hanging off any part of their bike. But they also don't use brooks saddles. The average brooks user likes to have a carradice or similar
so marketing wise I'd play up these features:
-never be without your toolkit again
-save room in your other bags
now, the biggest problem for your product will be the seat flex issue b/c nobody wants metal bits putting pressure on their soft bits.