specific for my experimenting
For that purpose, I see nothing wrong with the Idea, but keep it tight. If you end up with lines between components (trace to connector, backplane trace, trace to other component) of any noticeable length you will be in trouble. The PC bus is based on buffered signals which alleviates a lot of trace length concerns. Going that route with a '51 design is probably not appropiate.
I do, in my industrial applications, occasionally use a daughter board (sandwich style) one example being that I have a unit that is identical but for being available with 3 different serial interfaces. There the serial hardware is on a sandwich board approx 1*1"

Erik