There is now a standalone APRS digipeater firmware available for MicroModem! This means you can now use MicroModem as a digipeater without any computer or other host equipment connected. You can grab the sources from the MicroDigi GitHub repository.
The firmware should still be considered beta quality, and there might be bugs here and there, so watch out! You will also need to configure and compile this yourself. To do so, set your callsign and SSID settings in the "config.h" file. There is also other settings you can configure, such as path clamping and whether to operate as FILL-IN digi or a regular WIDEn-N digi.
Please report any bugs you find, and also let me know of feature requests!