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Have you referenced this white paper discussing custom data references? This talks about the x86/x64 case in the Registering Data References section using regplugin.
Specifically, "If you have built your data reference targeting x86 or x64 platforms instead of AnyCPU. You can have the dlls in separate file locations and register them with RegPlugin by referencing the appropriate paths."
Is there an error you get when trying to register your plugins? What is stopping you from building your plugin for AnyCPU?
Also, building a custom data reference to read data from PHD seems like overkill to me. I think the typical route is to migrate PHD data to PI using OPC methods to move data.