You can adb shell into it. You need to install the android debug bridge on the main openwrt image and make sure that you do not have the /dev/ttyUSB* nodes you get through the "option" usb to serial driver (because ADB talks to the USB endpoints via libusb).
I looked around a bit in it previously. adb is running as root, so your chances of being able to change (and break!) something are pretty good. Post number #102 in this thread has some instructions on how to run commands in the adb shell to talk to the modem behind the modem. If it'll allow you to change the IMEI no idea. The IMEI isn't supposed to be changeable in the first place.
I haven't found anything noteworthy on that mini-android though. An earlier comment suggests that there's a HTTP server running, but I couldn't find any open ports. It seems to use an ARM CPU with 32 mb ram. The modem in the modem appears to be MBIM (rather than QMI) based. I concluded that whatever I can reasonably do on there I can also do via the uqmi command line tool or mmcli and decided to free space by removing adb again.