[MarshalZCC]

I ended up making one for my community, but I'm not sure whether its considered a reserve slot or not:

I increase my max slots by 1 and hide that extra slot. If that slot gets used, once they are authenticated I perform the following: if they are a donor then I kick a non-donor; if they are not a donor then I kick them.

I don't think this is reserve slot in the sense as sourcemod reserve slots work. Its more of a dynamic priority management system. You could say it is a type of reserve slot system though.

If people are interested enough, I could try and publish something that everyone would be able to use.