Wow, that worked great on my test PDF. This made me realize that I didn't provide full details in my question. First, can this also be done so the field can be modified in the form to write to the Custom property? Second, here's the crappy part of this, I need to map this on a locked-down government form.
I am using SolidWork PDM (SWPDM) to map a SWPDM variable to the Custom properties in the PDF. While I have gotten this to work, I could not get the Custom property to poplulate the form field. Your solution works great, but I can't edit the government form that was created using LiveCycle Designer.