I don't see how a rev-limiter will work with a manual transmission vehicle though which is directly connected to the crankshaft once the working clutch system is engaged.
While this is an over-simplification of a system that seems like it could work with a small tachometer monitoring processor, small servos could be mounted next to each gate that would rotate a bar 90 degrees after the engine speed is too high to safely allow that gear to be engaged. The bar could have a "fork" on the opposite side of the gate for mechanical stiffness. There would be one servo and "fork" for each gear gate other than high gear or maybe the two highest gears. This means either the servos would need to be really small or the gates would need to be spread really far apart. Gates far apart are generally not preferred for performance shifting, and if you're not performance shifting, you probably don't need the gate lock outs.
I alternative thought about a sliding bar system and a single linear actuator that would just slide a single bar for the lower gates and a duplicate system that would slide a bar for the upper gates, which solves the issue of needing the gates to be spread apart, but replaces that with a need for a wide distance between the transmission tunnel and the driver's leg or seat to accommodate the fully released bars.
Hmm, looking at the clock, I wonder if this is similar to the long response Stealthee wanted to avoid.
-Rod