I've found the BLM more than willing to answer questions. If you contact them with the freezebrand number they will be able to provide you what HMA the horse is from, when the horse was round up from the wild, when it was adopted and titled. If you go to this site: Wild Horse and Burro
there is a "Contact us" link on the left hand side.
Once you find out which HMA the horse is from, you can go to this site and find out more information on each one and see pictures of other horses from the same HMA: Herd Management Areas
There are no registration papers for BLM mustangs. The original adopter received a title after they had the horse for one year, and most will pass that on to the new owner, but its nothing that gets transferred and documented like a registry. If you had gotten the title it would have provided the information above, but you can get most of that by contacting the BLM anyways.
Would love to see pictures of your mustang!