I totally understand what you're trying to do now.
You've hit a limitation on the tool that isn't commonly hit but is being address in a future version. There's currently no way to make a model that can be used by multiple dialog workflow components so that you can share some common forms.
There is a very complicated option that you could potentially use: External Dialog Step.
This step allows you to temporarily pass control out to another aspx project and wait for a redirect back to resume the workflow.
You could build your common handling in a web forms project and then use the ExternalDialogStep in the dialog workflow components to hand off control and await the return.
Warning: this step is fickle and complicated and I don't really recommend it, but you *could* do what you're trying to do with this and not have to wait for the next release of Workflow.