There is no big difference between BF531 and BF526, you must change only the blackfin.cfg file...its a textfile with the peripheral registers of the blackfin.
In the DS1052 v2.05 are some functions completly removed, so you have no chance to activate it without firmware patching. On older versions, i don't know.
I'm sure it's possible to hack any firmware version of the Rigol DS1052 to make 100MHz. The only question is how. If you have the firmware, you can find a way....