I have not found anyone that has attempted any sort of real attempt to overclock a Duron 900. As AMD is always tweaking the production processes, your chance of achieving a stable 1.2Ghz seems entirely possible. The stepping code on the Duron 900s that I have seen is 'ANCA', and there is some anecdotal evidence from a few review sites that this should be a reasonable overclocker.
It will most likely not turn out to be in the 50%+ range. Something along the lines of perhaps 35%. It appears that with the relatively small price differential, that most are opting to overclock T-birds, yours truly included.
T