It is not currently possible to add unbound server rules to your form / view.
Going forwards when you then create an unbound server rule, you could limit the rules that you could actually execute to match the set of rules that you can call from the bound server methods. You could also limit the calling of that unbound server rule such that it can only be selected in a bound server / unbound server rule.
This would allow users to still create 'clean' rules - I have rules on all my views for load data, save data etc.
I always call the unbound load data rule on view initialize - it calls a SmO method to get the data for the view and populates the view.
If this could be done server side then that would be nice and would benefit from the performance bump.