As the title says, is it possible to decode the base64-encoded contents of my PDF back to an XDP format?
In your form if you have bound the webservice in Data View tab then you shuould be able to pass that token as part of the web service request data. By doing this you are putting the token as the form data so you might have to populate the value in a hidden text fields in the form and thus the value gets stored there but is not visible.
To answer the base64 decoding issue, you may be able to decode the data via java api. Have a look at package com.adobe.idp.common.util.Base64 class in LiveCycle client apis and there is a decode method there.