I have added support for aesgcm:// links on both iOS and Mac clients. There is still a bit of optimization to be done but it should be in the next client releases. It appears that the documentation for this functionality is wrong which made things a little tricky. It turns out the IV is 16 bytes and not 12 as it is written. Additionally unlike what the xep says, OMEMO uses GCM 128 and this uses GCM 256 and has a different key length.