We use proxies in many situations. My neighbors recently provided a humorous example of a proxy. They decided to build a diorama of log reindeer after they could not induce the local deer to hang out in their front yard and pose. Perhaps because I let the dog out to bark at the real deer. The log deer erected were a proxy for real deer and, in this case, not only will the proxy suffice but they might be better for neighborhood peace.
Naming a proxy for a product owner in an agile project will be far less satisfactory than a herd of log reindeer. A product owner acts as the Voice of the Customer channeling information about how the product will be used to the project team. This information is critical for the team to maximize the value of what they are building. Product owners also provide the team with priorities and decisions based on their knowledge of the business environment. The further the person acting as the product owner is from the financial responsibility (profit and loss) for the product or the day-to-day operations that use the product the lower the fidelity of the information he or she can provide. Poor information or slower decisions will yield lower value to the organization regardless of what is being built. While a proxy product owner might not be a log reindeer, the further they are from the business the more apt you will be to get termites rather than the answers your agile project team needs.