In case anyone is looking to automate deployment of Nintex solutions (and why wouldn't you?!), you can visit my blog to get a PowerShell cmdlet that imports UDAs into an environment, and retains their GUIDs so that workflows that reference the UDAs also import seamlessly!
First of all just want to say thanks for the useful script. Can I please ask you what you have to get an imported workflow referencing the UDA to appear in the Analyze Dialog (which shows workflows referencing the UDA and thus allow republishing of the workflows referencing the UDA).
In my case the import of the UDA via powershell worked great. When I imported the workflow that was referencing the UDA and published it, the workflow and the UDA worked as expected.
The only thing I was not able to work out was how I could get the workflow to appear in the Analyze Dialog. Any feedback would be awesome.
Edit: I also noticed by looking at the dbo.UdaUsage table that the published workflow referencing the UDA does not appear in this table. However if I open the workflow referencing the UDA in the workflow designer and republish it, it seems to then appear in the Analyze Dialog for the UDA as well as in the dbo.UdaUsage table.