The Shafi'is and other students of knowledge will be familiar with Imam Shams al-Din al-Ramli RH.
His father is Imam Shihab al-Din al-Ramli and he was a student of Shaykh al-Islam Zakarriya al-Ansari. Needless to say he is a massive authority for the Shafi'is
The shaikh wrote a book where he collected some of his father's fatawa.
The Askimam site has a short biography of the shaikh HERE for those who require more information about him.
Anyway, I've been researching this issue of Istigatha and I came across this interesting fatwa of his:
i shall try and paraphrase what is mentioned
the question is essentially that the awaam when in distress say "Oh so and so" and "Oh RasulAllah" and things like this from istighatha with the prophets and the awliya the ulema and the rightoeus. Is this permitted?
and he replies
بأن الاستغاثة بالأنبياء والمرسلين والأولياء والعلماء والصالحين جائزة وللرسل والأنبياء والأولياء والصالحين إغاثة بعد موتهم ؛ لأن معجزة الأنبياء وكرامات الأولياء لا تنقطع بموتهم
Istighatha with the Prophets and Messengers the Awliya, `Ulema and the rightoeus is permitted
he then says that it is amongst the mu`jiza of the prophets and the karamat of the `awliya which does not cease to end with their deaths
basically he then goes on to mention how the prophets are alive in their graves
then he talks about the karamat of the awliya not ceasing after their death
[note: the views held by the Imam are his and I do not necessarily agree with or disagree with his view]