As a matter of general principle should the shaft and bushing be made
of different material? What would happen (for argument's sake) if one
spun a brass shaft in a copper bushing?

Michael,

As a general rule of thumb, same-metal contact under pressure and motion
will cause galling; the softer, the worse. Copper on copper would be a
very bad bearing selections, because not only is the metal the same, it's
soft.

Steel on copper works. Brass is a good bit harder than copper, but
contains enough copper that I think it would smear off particles and
gall.

Lloyd