It would make sense to assume that it's on Dreamcast because all of RSG's previous games were on DC, but why would they try to hide the platform if it was another DC game?According to statements, the project will test wide-ranging possibilities for both the industry and the title’s chosen platform, which has never before featured a project of such quality and scope, and is sure to be closely watched by other developers and publishers. Should the title be successful on this system, it may usher a in new wave of quality on the platform in general. Indeed, the platform was chosen specifically to test its possibilities – and stretch its limits.
The part about stretching the chosen system to its limits sounds promising though, considering all independent DC releases have been 2D. On the other hand, this might suggest that they are developing on a more primitive system, because how could they possibly push the Dreamcast more than games like Shenmue II?
Voice acting pretty much confirms that this game will be on a disc, so that limits the choice of platform to Dreamcast or Jaguar CD, unless they've figured out a way of getting past lockout on a different disc-based system.