VooDoo asked for native serial.device for classic Amiga builtin serial port. I think i can help a bit here.
Did you try to compile an example which comes with MorphOS SDK? It looks like it's exactly what you need. It's limited to 8n1 mode but it works!
If noone built this, i can try to do this. In case of success it will become one more package for Unmorphos project. But, have to warn, sorry, i can't actively maintain the package because i have neither time nor hardware to test it. So maintainer will be wanted.