I think that this mode S protest is a bit of an over reaction from AOPA as it is clear that the JAA has not the first idea of the technical problems introduction.
Mode S is only of eny benifit if more than an SSR reply and alititude are transmited , the mode S should also transmit HDG and IAS.
Apart from the power supply issue how is the mode S transponder going to get the IAS and HDG imformation as this is required in a digital format ? has your aircraft got a digital pickoff for these systems ? i think not !.
As i fly IFR some of the time i have had to give some consideration to all this and and have held off from fitting the latest very cheap JPI fuel flow transmiter because it MAY get in the way of the most likly mode S instalation.
At the moment the best way of getting the digital output for the mode S is the Shadin fuel/air data computor apart from the fuel flow it takes the HDG imformation from the HSI bootstrap (28VAC/400HZ power required)and pitot/static from the aircraft system and it also has a OAT probe and converts this to a digital signal for the mode S HDG ALT and IAS modes.
The up side to all this is if you have a good panel mount GPS (king KLN90 / 89B and most of the garmin IFR units)this can intigrate into a system aproching the capabilitys of an modern airliner FMS system giving you imformation on actual wind speed and direction , TAS ,fual burn to waypoints,and more acurate altitude imformation for the APP mode for the GPS and all this via the screen on the panel mount GPS.
As you can see its a powerful system but it is also utterly impractical for most light aircraft that the JAA could not insist on it because without the aditional imformation from the ADC it has no benifit over mode C.