Hi,
the matte option or the render layer method are used to get the contact shadows (and more) from objects composed over a background (HDR or image). If you want to use a background image in C4D-Octane, just use a standard Background c4d object with a standard c4d shader material, put your background image in the color channel, activate the "Alpha channel" option in the kernel settings and that's it
This the only case where a c4d material is needed inside c4d-octane
ciao beppe