(for altitude only) No one said that there had to be a conventional static system (static port(s) on the side of the fuselage, on a wing probe, etc.). So, yes, your pressure devices mentioned above (watch, iPad, automotive unit, etc., IF baro adjustable) work, and IF … per the regulations (for certified aircraft) the unit meets the TSO for altimeters (static pressure in -->correct altitude) AND the static system (in your case the cockpit environment and the watch) must be calibratable to +/- 30' per 100 knots (airspeed).
I'll address airspeed in my next reply (to get your quote on that topic).