Wonder how "proprietary" this locking connector is, because this looks much alike the KNS-8400 locking cable, perhaps there's some uncommon standard of locking 2.5mm cables?
FWIW I had success using non-locking cables in KNS-8400, perhaps the same is possible for AKG - if the 2.5mm connector is very slim, you might be able to push it into the connector anyways, it just won't lock because it doesn't have any holes. Bose's 2.5mm cable from QC35-II worked that way, so maybe any slim 2.5mm is possible.
@solderdude did I read correctly that Sennheiser's HD59x line uses 2.5mm connectors? Maybe these would also work.