Increase RAM

Is it possible to increase the RAM size on the Beagle - through a
daughtercard e.g.

No. The only way to increase storage is via the SD/MMC connector or a USB thumbdrive. On a Rev Bx Beagle memory can be increased by replacing the POP memory, but it must be done by someone who has the right equipmen and it may mean that the processor will need to be replaced as well…