The bug report as originally stated was incorrect, although the problem still exists. The cause of the error stated was indeed that I was not using a leading zero for the dmode value. However, it is still the case that it is not possible to set the SGID bit with a dmode value of 02770. Looking through the dokuwiki code shows no errors that would cause this. And indeed writing a simple test script shows that the problem may lay outside dokuwiki's realm, unless a work around can be devised.
To sum up, mkdir can set the SGID bit, unless the umask prevents it. chmod does not seem to be able to set the SGID bit regardless of safemode. Thus, anytime that dmode is configured to a value that the umask will change, and thus dperm value is used with chmod, it is not possible to set the SGID bit. Is there a possible work around to implement in dokuwiki? Is this is a bug in php? A php configuration problem?