Sorry, I just found out that it's just a wording-problem!
What half of the tools/GUI's calls "custom attributes" is called "custom fields" in others,...
I asked my customer to set a sample with the PowerCLI-thingy and checked it in MOB.
MOB shows it as "customField" (?!?!?!) ...so I'm fine, I suppose
| availableField | CustomFieldDef[] |
...sorry to bother you with idiot-stuff like that! 
cheers
Sandro