v1.7.5.2 has now improved scheduled transactions, such that if you set a scheduled transaction for the 30th or 31st of the month, it will remember that as a "preferred" day. So if the next month has fewer days than this, the following scheduled date will revert back to the preferred date.
I think this is simpler than having a separate entry for "last", when you can simply specify as 29th, 30th or 31st as desired.
Currently scheduled entries automatically roll over weekends, but I will add this as an option in a future release to roll forward, roll back or not roll at all over non-business days.