Having just become aware of this BULSTAT issue via this forum, I sent an enquiry off to my Sofia based Lawyer, who I used when buying my apartment in June of 2005.
His reply was that the BULSTAT does NOT affect any foreign national who bought prior to the date of inception of the BULSTAT law, 8th or so of Aug 2005 (I can't recall the exact date).
I did specifically mention to him that there was a general interpretation on this forum that foreigners who bought property in Bulgaria prior to Aug 2005 had 3 months in which to register, or face a fine. His response was that the registration within 3 months rule related to
"natural persons who are social insurers (i.e. who are obligated to pay social security payments for their workers in Bulgaria)"
he went on to specifically stated that the 3-month rule did not relate to foreign citizens who bought property prior to the date the law came into affect.
My lawyer is from a highly reputable firm from Sofia, and so I am satisfied based on his advice that I have no need to register with the BULSTAT.