Perhaps if the sub-controller pairs with the Move, rather than the PS3 itself, they could get around it that way, as only the Move would count. But I'm not familiar enough with bluetooth to know if this is do-able or not.
And yes, the PS3 is limited to seven bluetooth connections. You can confirm this in the controller settings, where you can change the order of connected bluetooth devices, from 1-7.
If you're wondering why they went with seven when few [if any] games would need more than four, it's for things like bluetooth keyboards, mice, remotes, headsets, and the Logitech Harmony Adapter.