Here's another idea I came up with while shaving the other morning:
A sliding valve that works similar to an electric shaver.. 3 steel plates with many small slots in them -
The top one is stationary, and the slotted area covers the entire top of the combustion chamber (except for the glow plug). The intake and exhaust ports could each cover half of the combustion chamber except for the glow plug area.
The bottom plates slide the distance of the width of one of their slots (very little movement). It's reciprocating, but since it doesn't move very far, it could be actuated by a very non-linear captured cam, thus requiring no springs.
Of course, the limitation of this design is that each open "port" could only account for less than 1/4 of the total cylinder head area, but there would be very little time spent opening and closing valves. Each valve's cam could also have an independent profile.
The other cool thing about it is I think the valvetrain would be much more compact than my other rotary valve designs. It would also have a very tidy combustion chamber with a centered glow plug. It even has the potential to be designed with a hemispherical or slightly coned shape - of course flat is easier to make...