Could you use one main workflow then have all the others linked back to it using the Related: Request field? A buttons shows up (in the sub-menu area) that shows all requests related to that one. Are you using Event Manager to create the workflows? If so, another idea would be to copy the PO# into the request then search by that field in Synergy.
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You could take this one step further by creating a custom ASP Page that would show all of the requests for that particular Purchase Order and their statuses. This would give you a visual display if the workflow is Open, Approved, Rejected, etc. This ASP page could be saved as a Favorite or linked to a Start Page with a Search for a specific Field in a request, Account, Date Range, etc.