Does the Quran or any authentic supplication mention the practice of using Wasila or asking someone (whether deceased or alive) to pray on your behalf?
Yes. We have in Quran many verses to prove that:
‘First: In Sura Yusuf, verse 97. The sons of Prophet Ya'qoub said : O our father, pray for forgiveness for us from our sins. He replied them : I will pray pray for your forgiveness. If it was not right to ask someone to pray for your sins, the Prophet Ya'qoub should have told them not to ask, and he should have not accept their request to pray for their forgiveness.
Second: Allah (SWT) in Sura An-Nisa;, verse 64 is teaching us in Quran how to seek forgiveness. ( Had they when they wronged themselves (by sinning) come to you and prayed for forgiveness and the prophet prayed for their forgiveness, should have Allah accepting repentance and Merciful.
These are just two examples from Quran. We have many authentic Hadeeths from the Prophet (SAWA) in this regard.
Wassalam.