How long do we have when night starts to pm you? Because however long that is is how long it will take to find out if someone has dropped out. What if someone goes Vla for the first half of night and can't be around at deadline(so they inform you before day end) and then they drop out because there v/la turned into a drop out and they don't inform you. Still going to have no idea they dropped out till mid to end of night.
I think the harder part of this is a situation in which a PR or mafioso has not PMed within the first 24 hours of a lynch occurring.
Nights run (generally) 48 hours. Given it probably takes a day to get a replacement (even with a substitution roster, you'd have to get that person a PM and a response), the PM deadline is probably halfway through night, or 24 hours.
If a lynch occurs on a Saturday, it's possible people don't see it until Monday. I PM all PRs that haven't contacted me at night at the 24-hour mark. That way I know they got a message from me (plus an email notification) regarding night actions. If they don't give me an order in time, unless it is a compulsory action, they just take none.
With yuma's system, instead of a PM at the 24-hour mark, you get replaced. I guess if that's clear from the very beginning of the game, it's okay, since everyone has fair warning.