Is there a "kink" for setting up a slitting saw to saw through
round bar stock along the axis, so that the slot is exactly
across a diameter? (I presume this to be a similar problem to
setting up accurately for cross-drilling)

This is actually a Ham Radio application; I wish to make
up some very large banana-type plugs from the 1/4" round
bar stock that I have in small quantities. For this, I will need
to saw two diameters in a cross shape, and then splay
out the leaves slightly to make a springy and rubbing contact.