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