(Hello. I'm newly registered. Otherwise I would have replied earlier.)
"Process Hopping" sounds to me a lot like the
Door concept in Sun Microsystem's research OS
Spring.
A type of Unix-ified Door mechanism was later added to Sun Solaris.
The Mill is a CPU architecture under development which has threads and
"Portals" in hardware. A portal call uses the same instruction as a local call: it is identified by the destination's protection bits.
Things get complex with nested calls and error states though. I think that what would could be most interesting to read about is how those issues were addressed.