Didn't
Circuit Cellar do something like this in a 
 series of articles?
   You're probably thinking of the Trump Card, which
 used a Z8000.
 (Z8001 in particular) 
 
 BYTE/CC did feature a 68020 based accelerator or some such for the peecee. Now it only
shared the bus, but sat on it nevertheless. What's the difference I ask?