Intercompany matching at Parent entity ?

I haven't investigated the possibility of doing this.  As a general practice, only base Entities should be designated as IC partners.  This ensures that the IC transactions between partners will eliminate in every hierarchy.  Also, simplifies future reorganizations of Entity hierarchies.  IC transactions between 2 base Entities eliminates at the first common parent.  You can certainly create a Parent Input Workflow to submit the Balance Sheet entries at different Parents.  If these are IC transactions that are submitted at the Parent Entity level, I don't believe eliminations will occur between the Parents.  Just in case you were expecting eliminations to happen between the Parents like you would see from the base Entities.  From the IC Matching perspective, the logic is run for base Entities.  The IC Method Queries used for the IC Dashboards are really flexible but I don't believe Parents are taken into account for IC Matching.

I would suggest submitting a ticket to OneStream Support and have a discussion with AAS on the feasibility of a custom IC matching dashboard for this process, if you have reached a dead end with this post.