Holacracy Community of Practice Archive, 2015-2019 Community Holacracy Web Site

There's actually no provision that would let you appoint anyone as a proxy, although you can allow someone else who is already invited to the meeting to represent your roles, under the terms in §3.2.2 and §3.2.4.  You could technically allow someone else to, but that wouldn't get them invited to the meeting so it wouldn't 

The potential policy you reference would be just fine though and then it would be fully allowed.  That's because the lead link can already invite extra people into the meeting as core circle members (under terms of §2.3.4), and any lead link authority can be delegated (or partially delegated) to another mechanism.