This should be possible. Dreambox games are WASM, so anything that compiles to WASM should work.
You'd probably use Emscripten to compile your C++ into WASM, and then use some other utility to build the ISO disc image (only real requirement is it's in ISO-9660 format with a "main.wasm" located in the root directory).
You can view a list of all of the functions you can import here, in the VM's constructor it links all of the functions that are exposed.