There is discussion about a Part 145 Maintenance Organisation holding a Part M Subpart G approval. My understanding is that the Part M Subpart G approval is vested in an operator's AOC and and whilst some of the continuing airworthiness tasks may be subcontracted to another organisation, they remain the responsibility of the operator. There certainly are organisations that hold an AOC, a Part 145 MOA and also have Part M Subpart G approval.
One of the essentials is that the operator has a Continuing Airworthiness Manager ("CAM") who must be acceptable to the authority. Depending upon the size of the operation, a CAM may be directly employed by the operator or subcontracted, but according to Part M Subpart G, should not be employed by a subcontracted maintenance organisation but may do so with the approval of the authority. Good, in'it.