You CAN recharge alkaline batteries but only with a special charger designed for this. The manufacturers don't like this as they then sell less batteries. In the UK stern warnings are given not to recharge alkalines at all - I'm not sure if other countries have these warnings - Germany is more 'green' and may have a different warning. I think the chargers use a charge/dicharge cycle to build up energy in the battery.
There are some 'rules' you must try to follow, one of them is to not alow the batteries to fall below 30% charge as they won't recharge very well if you allow them to fall below this. Estimating this is difficult so I usually recharge them after every few weeks. You could look at the battery voltage to estimate the remaining charge, however the battery indicator in the HP calcs is far too late to use for this.
Perhaps a small program could be written for the HP48/49 to estimate the running time and give an output of charge left (assuming fully charges batteries were inserted) an alarm could be made whe 70% discharge was reached.
Recharging only works 10 to 20 times (but thats 5 to 10 times more use than if you threw them away after one complete discharge).
Just like NiCads, you will need to keep a spare set of batteries to use while the others are charging.
Please remember: DO NOT RECHARGE ALKALINES IN A NiCad CHARGER!