I have not done this, but just came to me as an idea if you want to give it a go since this is still open.
Doesn't look like you can powershell the custom attributes (someone tell me I am wrong please). If that is really the case and you really want to use the Adaxes custom attribute, how about finding an AD attribute that you are not using right now and use powershell to put the pin in there for a moment. Once that is done, create a scheduled task in Adaxes for users. Have it modify the customattributetext1 to set it to whatever attribute you used in AD. It will copy the attribute over. You could actually set another step to remove the contents of the temp attribute as well. Set it to target the users you want and run it manually.
Its a two step process, but you get it there. I am interested in knowing if this helped or even worked.