The boiler can probably be fitted with an antifreeze solution, but it isn't as efficient in heat transfer as plain water...you'll loose 20% of the heat output, but if you do that, you shouldn't have to worry about it freezing if you shut it off. You'd have to drain the potable water piping, though and put antifreeze (RV stuff) in the traps of the sinks and toilets. You'd have to be careful if it had a water makup system, as if there was a leak, and you refill with plain water, you'd be diluting it and it would raise the freezing point. Course, you shouldn't have any leaks, but it is something to consider.
I don't see any problem with a washer standpipe as long as it is trapped, vented, and long enough per the standards.