#125 Untargetable by opponent

[surtur2]

For the card Shielding_Plax, the ability reads:

Enchanted creature can't be the target of spells or
abilities your opponents control.

A simple addproperty property="untargetable" won't word
here, nor will "untargetable-by-idcard" and its
derivatives, since these will also cancel out
beneficial effects you may wish to cast. Is it somehow
already possible to implement this? Or is new
functionality required?