I'm trying to do the same thing.
I was able to communicate with the printer with and without binding rfcomm, I'm not sure if it's needed. Basically, what I did was
to find the channel. There's two that appear, I used the one that showed "OBEX" (0x0008) in the protocol list (channel 4), then simply
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ussp-push MA:CA:DR:ES:S@4 source.jpg destination.jpg
I got the response that I was communicating, and the light on the printer started flashing, but then it flashed red and nothing happened. So now I'm stuck.
Another thing, I sudo bluetoothctl to pair the devices, but I don't know if that does anything.
Edit: Ok, now it works. Turns out there are pictures the printer doesn't like. I downloaded one from my phone to the RPi 3, and sent it. Success!