DOSBox is not able to load directly the BASIC cartridge.
Some modifications are necessary, so you can load it from DOS in order to load/save programs from/to disk
PCJRBIOS.COM fixes some of DOSBox's default interrupt handlers, and loads the PCjr ROM into segment D000h (BASIC part will be in D600h).
BASIC_XX.JRC is modified to reference segment D6000h, and instead of starting the interpreter it returns after booting, making it possible to boot DOS.
Thanks goes to VileRancour.
PS: With MESS I doesn`t have access to the content of the mounted disk image. So I am not able to load/save BASIC programs from/to the disk image.