In his remarkable work "Manh al-Makiyyah", a commentary on Imam Busiri's "Hamziyyah", Ibn Hajar al-Haytami once again mentions the validity of the practice of istigatha. He states:
"Then he took recourse towards humbled supplication, the manifesting of submissivness and weakness, and expressing of grief and sadness, seeking the aid (istigatha) of he who does not disappoint those who seek from him (al-mustaghithin bihi). So, he said, out of hope that through the blessings of his intercession through him - upon him be blessings and peace -- he would be cleansed of his excessive sins:
'Oh messenger of guidance, I seek aid by means of you, seeking the aid (istigatha) of one apprehensively troubled,
the soul being harmed by his state.'
Istigatha in the nominative state being the predicate of an omitted subject i.e. masu'uli, and it is the calling out of he who seeks to be redeemed from hardship, or the lessening of it. And in the accusative state being an absolute verbal noun (maf`ul mutlaq) i.e. 'I seek by you a seeking...' namely 'I call you with a calling.'" (pg: 645, Dar al-Minhaj)
I think this should clarify the following:
1. Imam Busiri and al-Haytami both consider calling the Prophet (Allah bless him) directly to be valid,
2. al-Haytami clearly refers to istigatha as being a form of tawassul, defining the former as a "call" to the Prophet (Allah bless him). This clearly demonstrates that calling the Prophet (Allah bless him) directly in such a manner is a form of intercession.