Although it seems logical to prorate it, I've never seen anything that allows that to be done.

Material participation doesn't automatically make it a business; material participation it makes it non-passive.

Startup costs are for an "active trade or business".  I suspect that many people view short-term rentals with Material Participation as an "active trade or business", but circumstances and opinions vary and I'm not aware of any 'solid' bright-line determination for that.