Unfortunately Documentum only provides a repeating attribute called r_version_label. This attribute can contain other values such as CURRENT or other user entered values.
But the important thing to note is that Documentum always stores the version number in the first r_version_label value.
Based on this assumption you can retrieve the version number from an IDfSysObject using the following ...
IDfSysObject.getRepeatingString("r_version_label", 0);
Hope this is helpful.