Or maybe that would too much of a hack... What do you think?

I can try to take a look at the API docs and see if Telldus have added some info about how to handle this as well when I get some time.
Moderator: Telldus
Seems you're right. I can't get the API explorer to clear the name of a sensor either. If I post a empty value using the Telldus Live! web page it does set the name of the sensor to "(no name)", but as soon as a I refresh the page the old name is back.