If you're healing main spec. Why not just go as a healer and take it? As a DPS I'd be more than willing to blacklist any dps that needed the robe of their offspec when the groups healer needs it.
It does kind of peeve me that you're reserving an item for your offspec (in terms of what role you are filling during that run). But just make it clear ahead of time. ML shouldn't even be necessary, and if it is just state the reason and that the mount will be rolled on (big) if it drops.