There is no hadith exists in 6 sahih that show tawassul through Prophet(SAW) after he's passed away is prohibited (ref to Tirmadhi ,the blind man hadith). And I don't see the reason why we shouldn't consider other books of ahadith as truth when the chains are mutawatir and isnad are sahih.
Also, when we do tawassul we do it through the good deeds of a pious man(dead or alive) not for his life or death.
I am not saying that it is prohibited or not, what i am saying that it was not practiced among early muslims.