Let me preface this by saying that it is in no way intended to be gospel truth, but is the best information I can find.
The order to start using Zimmerit was issued on 29 December 1943 and the order to cease using Zimmerit was issued on 9 September 1944. Therefore, we can assume that most (if not all) Brummbars produced between about January 1944 and September 1944 had Zimmerit applied. Some units continued to apply Zimmerit for a period of time after September 1944 anyway.
298 Brummbars and 8 converted PzKpfw IVs were produced between April 1943 and March 1945. Therefore, there were a significant number produced after September 1944 that probably would not have had Zimmerit applied (although some may have). I don't have the exact figures on the number of vehicles produced between Sep 44 and Mar 45 or the units to which they were eventually assigned.
Robert