You can utilize the electric convection heaters base board is a problem in a shower stall ;^))));

You can utilize a a pipe wrap of heat trace (that would be left on much of the time because they can't really thaw a frozen pipe in any cold); or,

You could consider adding valves, drain down the upper, unused areas; and, if necessary, blow-down the lines if they don't drain properly).

Exterior shower supply risers are a challenge. I'd either heat the space to some level -- or just drain it down.

Heat trace with insullation over the pipes is pretty efficient. But I'm not a fan of those inside walls in living area. In any event, make sure if a smoke alarm goes off up there, you can hear it below well enough to wake the (not to be) dead.