If you look at the schematic for that board, you can feed in a DC voltage via a diode on the board using an isolated power supply to keep the RAM contents intact while you replace the battery.
Since some models (including mine) do not actually have a separate battery (battery is embedded in the RAM), could a 5V backup battery be "permanently" added using a couple of 3V lithium cells, a voltage divider and a diode?
Assuming its using the same as 2465B, then the cal data is stored in the battery backed up SRAM Dallas DS1225Y, then no, the lithium battery is potted tightly with epoxy with the sram chip in there, and there is no external connection for the power source.
The only safe way is to desolder the DS1225Y chip, copy the calibration data, buy a fresh one, copy back the cal data and put it back on board. Digikey has fresh DS1225AD (DS1225
Y replacement), just bought few months ago with batch code dated 2012.
Fyi, the cheap MiniPro TL866 Universal Programmer as reviewed by Dave officially supports this chip, mine reads and programs this chip flawlessly.
Pic of my A5 board, the BBSram is the black rectangle box labelled Dallas DS1225Y.