You wouldn't have to tell the receiver anything it would just work because that is what it is expecting, for mark parity.
Yes I have come across this sort of parity before when I was working with access control systems but it is very rare.
So yes the simplest thing is to send two stop bits.
You could write your own software serial routine to do this or modify an existing library.
I think it was the other way round. Mark is the un-driven state of the line and Space is the driven state. Leading to my favourite asynchronous serial data malfunction where the line is driven continuously, being knows as "spaced out".
That is why in TTL format, mark is a logic one, because TTL inputs naturally float high and you have to drive the input low.