I would like to have more storage available for running local applications and services. I have been thinking along the lines of mounting an extfs formatted microsd card mounted as part of the root filesystem (e.g /opt), and if available, allowing packages to be installed there. something along those lines anyway.
Has anyone done this, is there a guide for this? I was thinking about maybe unionfs or similar?
You might like to look at this OpenWrt Page that talks about using extroot for extending the program space:
The MT300A router (now discontinued) had an internal daughter board for an SD card.
The MT300N-V2 has the same connector pads (9 pins) on the board, so it may be possible to install a daughter board.
One note of caution is that these small devices run fairly warm as there is no heatsink provided on the MT chip and no ventilation of the case, and so the SD/USB memory can get quite warm, potentially leading to premature failure.