This might be of interest: a thread about bringup of a 68000 co-processor for a BBC micro. The co-pro will be a standalone SBC, with CPU, RAM and boot ROM, and an 8-bit port to the host machine from where it receives filesystem and i/o services.
The relevant files seem to have been archived.
Looks like there were at least two 68000 copros at the time: a Torch one and this Caspar one.
See http://stardot.org.uk/forums/viewtopic.php?f=3&t=2920
Seems the Torch board ran a Unix. See
http://chrisacorns.computinghistory.org ... 68000.html
The thing about Acorn's Tube interface is that physically it's just a small area of the host's (6502) address space. So any 8-bit hardware can be placed on the other end - it need not be Acorn's own Tube chip. If it's not the Tube chip, then some hardware specific support code will be needed on the host side, either as a ROM or loaded from disk.