yup, just did the same thing at my FFL. I transferred my lower from me, to myself as trustee of my trust. FFL just did the transfer under my name then I filled out a receipt that my lawyer did for me that I, muddywings am accepting AR15 lower S/N 12345 as Settler and Trustee of muddywings trust.
I paid the $35 bucks, had them print out the CBI report, I filed it with my trust and added the lower to my schedule A.