pontoon speaker quandry

I have a kind of somewhat unusual situation that I thought perhaps someone might be able to offer some insight or guidance in this.  Let me give you some of the details and backstory 1st.  54' Pontoon boat that has a front "porch" the inside living quarters, a back swim deck/platform, and an upper deck lounging area.  The customer wanted two completely different audio/video systems one 120 volt A/C and the other 12 volt DC.  We are doing the 12 volt system which will be used a good majority of the time.  For the  system I am using the JL Auio CL-RLC Line Driver that is than connected into the JL Audio XD600/6 6 channel amplifier.  The customer was adamant about not having a head unit and so to accept various device inputs i dug up an old alpine switchable audio/video switcher that will source thru 5 inputs.  We installed 3pair of Boston Voyager indoor/outdoor speakers (all 8 ohm) and 1 pair of Alpine SPS-M600 (4 ohm) upstairs.  Now everything works great and customer is thrilled with the result however he calls me and wants to know if it is possible to have the ability to either control volume or simply turn off certain speakers.  Also there are a bank of switches that did this using an old Sony deck but from deck power not amp powered, and all these switches did was to open the negitive side of the speaker output on "off" and when switched "on" would complete the circuit allowing the speaker to be heard.  I called JL and they like me thought not a good idea to use the switches either due to what possible effect might happen to the various channels being cycled on/off so brutally and not sure of the switches ability to handle the AC from the ams output.  Yes I know a impedance matching speaker selector with seperate volume control be the answer but I have not found one that is 12 volt only.  I have tinkered with using the imput signal and also possibly using the remote controlled bass unit that can control channels either channels 5&6 or channels 3/4 & 5/6 so you can see my issue.  Customer wants to be able to control 4 zones individually.  Anyone have any ideas?