There is a 9 page datasheet which tells us that the main application core is an 1GHz Cortex-A5 core. But that the modem includes a "RDA RISC Core 32x32 bits" cores. One for the System CPU (referred to as XCUP) and Baseband CPU (referred to as BCPU).
A partial pinout for this SoC can be found in the Orange Pi i96 released schematic as it also uses the RDA8810PL SoC, while omitting GSM and GPRS RF hardware.
A cross-compiling environment from RDA Micro can be found in the archive Orange Pi 2G-IoT Android SDK Source in it's root one can find "modem-cross-compiler-linux.tar.gz" and the "modem" folder with some stuff. So it might be possible to write and compile our own code for the baseband processors and then build usable images for flashing the baseband with them.