Not sure how it will interact with this, but at least one of the compatible Yubikey devices that was mentioned also supports NFC specifically for use on mobile devices that lack USB ports.
As an addendum to this, after looking over the spec, it appears that the FIDO-U2F protocol currently only supports USB, but that support for NFC and bluetooth is planned in later versions of the specification. Presumably a firmware update sometime down the line would be sufficient to bring the Yubikey device up to date with the NFC standard and allow it to work over NFC in addition to the currently supported USB version of U2F.
Yubico doesn't allow firmware updates. They do, however, support the GlobalPlatform standard for uploading new JavaCard applets, and, if the NFC bindings were designed such that a JavaCard applet could implement it, then a U2F applet could be uploaded to an existing YKNeo.
