Will a port pin that is an "open drain output" support a SEROUT command?
Yes for sure, use Dave's suggestion.

The pin in question RA4 is multiplexed with a Timer0 module clock input. Can it still be used as a digital pin?
Yes

Will a SEROUT command set the pin digital output even if the pin was previously set as analog input?
Nope, this is also why we see many question about that. Always make sure you disable multiplexed stuff(when possible) on an i/o before using it.

ADCs/analog comparator are popular.