Jxk - Pilot-owner maintenance forms part of the aircraft's Maintenance Programme. The management of the Programme is a Subpart G activity but it's likely to be developed with the input of the involved Subpart F companies. You're quite right in saying that the person performing the maintenance must be judged competent (AMC 803). Add to this that they must comply with the Programme and the Part M maintenance standards (Subpart D). What the AMC doesn't say is how competence will be judged...
The scope and limitations of what a pilot/ owner can do is well defined in Part M (Appendix VIII). No AD searching etc is required for these tasks and in the case of a group-owned aircraft the Pilot/ owner could be anyone in the group, so long as they have a valid licence (MA 803a).