Actually, a modified version of the * approach will also work.  When switching processes, swap the whole process out to your fastest device (on * this was a single write to the drum) and swap in the new process.  * hardware had a bounds register, so it was only necessary to swap out enough of the previous process to fit the smaller process in.  So after a while, core started to look like an onion, with the current process at the bottom and pieces of larger non-current processes above that.

(I thought that * was MIT CTSS, but I can't confirm that.)

As for interrupts, the stock 2e has both IRQ and NMI lines. Erann Gat aka Ron Garret explains in <https://www.atarimagazines.com/compute/issue9/030_1_THE_25C_APPLE_II_REAL_TIME_CLOCK.php> how to make an external clock and hook it to the NMI pin for 25 cents in 1981 dollars (about $1.67 today; h/t measuringworth.com). 

John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
A mosquito cried out in his pain,
"A chemist has poisoned my brain!"
The cause of his sorrow / Was para-dichloro-
Diphenyltrichloroethane.                                (aka DDT)