As ET-Generation is shutting down their servers end of January 2017, I was wondering if there would be a possibility to backup the ET maps on the mania-exchange servers.

That's a fantastic idea!
Sad to see ET go... but glad to see you're thinking of keeping the maps accessible.

I'm not a member of the MX staff anymore, but I don't see a problem with this. We backed up other stuff from time to time, and this is certainly within the purview of TMX/MX.

It's just a matter of handing it off to the right person who has access in order to upload the files. TGYoshi is probably the go-to person for that sort of thing.