Just thought this forum might be interested in John Kortink's recently completed 6502/65816 project which takes the form of a second processor upgrade kit for the BBC micro:

http://web.inter.nl.net/users/J.Kortink ... 2/what.htm

It isn't an open design, but there are interesting features which might act as inspiration. For example, the 512k of RAM is available to the processor in flat form if it is a 65C816 or in banked form if it is a 65C02.

For the detail, you have to read the text document inside his software download.