The speaker selector box you are describing would be analog so there would be no difference between a unit designed for the home and one designed for a boat. I guess there are some advantages to this approach but I am hesitant to push the kind of power I will be running to the tower through my boat speakers. Most tower volume controls I have seen are on the input side to the amp (ie. PAC-LC1).
On my next boat I am planning on using a pre-amp EQ/XO unit with a fader to the tower so I can fade between the boat speaks and the tower, but you are right this requires an additional amp for the boat.
One thing to keep in mind is that the tower speaker impedence and the boat speaker impedence should be identical if you want to A/B them off the same amp. It's a pretty good idea if you are running something like four 4 ohm boat speakers and four 4 ohm tower speakers. This seems like it would be a common situation for most stock or dealer installed systems.
Something like this should work: http://www.smarthome.com/9725780.html http://www.hiddenspeakers.com/product/268
Consider I use a cheap $100 amp for my boat speaks a simple toggle on the amps turn on wires can accomplish more for less.
Good idea in the right situation!