You don't need a dedicated HORSE clock. Any Mac-compatible tournament clock that can display antes can be used for HORSE (or any limit game).
For the limit stud game levels (Razz, Stud, Stud8), just list the program's ante as your tournament's ante amount, the program's small-blind as your bring-in bet, and the program's big-blind as your completion bet (and small bet). Calculating the big bet on 5th street and later is easy, it's just 2x the small bet. The levels that use board cards (Hold'em, Omaha8) just use the program normally.