Related to your Query for improving the performance of PDFG using a Cluster, you could easily set up a LiveCycle ES/ES2 Cluster with the PDFG Component. The PDFG as a component can be accessed from Workbench when you have the cluster mounted either horizontally/vertically on different or single machine environment and once configured it would work in a round robin fashion to cater to the hits accordingly to improve performance.
The Main query if I understand correctly is to have a connection from Workbench to the LC cluster running the PDFG component, you could easily do that by installing a Http Server and configuring the Horizontal cluster to accept connections from the Http Server.
I can give you a example with a Websphere Horizontal Cluster running a LC ES instance, where in you have the node1/node2 running the ApplicationServer1/ApplicationServer2 instances on different machines. you could install the IBM http Server over the deployment manager that would have all the required configurations to handle the load.
The Documentation available on the Adobe website is very helpful. Please refer the link below.
Please let me know if the details helps and incase you have any queries, do revert to me, I would be glad to assist you further.
Please refer the link that I told you to refer so that you would be able to go through the cluster setup details. Adobe Provided lot of documentations with step by step guidance to setup the environment according to your requirement. I reckon the Document should help you in setting up the required environment.
As an example, you could contact Adobe Support, they would guide you on with a probable example that they can find with any prior history of this kind of setup.