Below the bar are some threads re interrupts less frequent than clock/12/65536.
Working with something else a solution came to me.
Using the PCA in the Philips P89C51Rx2 or the Philips P89C66x you can use timer 0 as input to the PCA.
With timer 0 in auto reload (8 bit) you can make the interrupt happen every clock/12/256/65536.
With timer 0 in "manual reload" (restart in an interrupt) you can make the interrupt every clock/12/65536/65536 which should satisfy all cases.

Hope this is useful,

Erik