I haven't heard any SMG-specific clutch replacement stories either. The SMG transmission in the Z4 is the 6-speed manual box with an electro-hydraulic control system attached, so in terms of load-bearing components it's unlikely to be more prone to problems that the standard car. (That said, I had part of the control system replaced under warranty following a failure, but that was at 20k miles so unlikely to be due to wear-and-tear.)
However, given my experience of how the system controls the clutch, I'd say it's more likely to facilitate a hard life for the clutch than a careful driver of a manual would. For example, it slips the clutch rather than downchanging to first more often, and performs more savage changes at high revs than most manual drivers could. But then these factors can be mitigated by driving style, and manual drivers could be performing clutch-dump launches on a daily basis, so there's probably not much in it.
In terms of longevity of the manual clutch; I've heard that some cars have had replacements as low as 35k, but I haven't heard of many replacements overall and most pre-facelift cars will be pushing 60k miles by now.