If the radios will work at the lower voltage AA rechargeables should work fine. However, I doubt that the desktop charger will charge them. If the voltage gets too low the radios will reset (turn off and back on) after transmitting.
The lower voltage will reduce power output, but AA batteries should last much longer than the Midland battery pack. The power output usually doesn’t affect range too much, and the ability to use the radios on the high power setting without quickly draining the batteries may give you more “useable” transmit power.
The Midland rechargeable battery packs do put out six volts – they use 5 AAA batteries (1.2V each) to keep the voltage up. The battery pack in my GXT850 can’t maintain 4.8 volts when fully charged during high power transmission, so using AA rechargeables may be an improvement even though the rated voltage is lower.