Hope I didn't get it wrong, you can try the iPart.FindRow， which will look up the parameter in iPart factory. Attached please find a sample I made. Noticed that we didn't add the extension (.ipt) in the rule.
I created two different style in the sheetmetal iPart which varies from its thickness, if you would like to share your sample I can look at it. you can send it to my email jingyiDOTliuATautodeskDOTcom.
i = iPart.FindRow("Test iLogic and Sheetmetal-iPart:1", "SheetMetalRule", "=", Thickness)
Quality Assurance Team