I dont know if this has been discused before but I have some doubts.

Can you use Maester of Suns ability to save Red Viper even though he is immune to character abilities?

Rozy said:

No, the FAQ states that "The effects of a character ability cannot be applied to a card that is immune to character abilities," so, even though Maester of the Sun's effect ("Response: If it is Summer, kneel Maester of the Sun and discard a card from your hand to save a HM character from being killed or discarded from play.") does not choose the Viper, its effect would be applied to the Viper and immunity will prevent that application.