I've done some reasearch on arduino, atmel and 9 bit serial. Again my goal is to stay as much in the Open Source toolchain as possible so that I don't have to support any more than I have to, and so my result is as standardized as possible:

• The arduino serial can not handle 9 bit transactions. :(
• Atmel, however does support it. "1 start bit • 5,6,7,8,or9databits • no, even or odd parity bit • 1or2stopbits"
• Looking at the existing Atmel serial library it is sadly hardcoded to 8 bits.
• Theres a bitbang serial library for arduino called New Soft Serial. It does not do 9 bit either :(. It seems like it would be not too terribly difficult to alter that code though.

So my options are

• a fresh atmel library, which arduino people wouldn't look at.
• A fresh arduino hardware or software serial implementation which I may be able to get upstreamed to them,
• altering the existing NSS product and attempting to get that upstreamed.

For now I choose altering NSS as it will be the least path of resistance. Here goes nothing.