The ICT 1301 had up to 100 'indicators' specified by two BCD digits in the instruction. All could be tested with an unconditional jump and some could be set and unset by program too. Number zero was always on to give an unconditional jump. There were ones for equal, greater and less as well as overflow, parity error, front panel switch states, as well as various peripheral statuses. Does that count? Not quite the same thing but IIRC the Elliott 920A used core location zero as its program counter. To be more accurate it used 0 by default when not using interrupts. When using interrupts it used 0 for level 1, 2 for level 2, 4 for level 3 and 6 for base level code. It also used those addresses plus one for its modifier (B) register. This speeded up interrupt processing as no need to save those registers.