umbracoNaviHide
is not supported in Umbraco 5 as per the words of Niels Hartvig:
While these special aliases does [sic] [recte: do] a great job and are super easy to use
(albeit totally impossible to discover if you don t stumble across
docs that mention their usage), the problem with these are that
they re magic strings which really is a mess (read: They re hacks
inside the core).
So they won t come back in v5 in the form we know from v4.
So, besides the Linq imitations being somewhat broken anyway, the short answer is that this (either form) shouldn t work (nor should Athul s answer).
The long answer is that you could use this property (and others like it) only if you explicitly support it as part of your Document Type. There is a feature request, though, asking for the built-in umbraco...
properties here should you care to follow and support it.
I would personally ask that you don t, however; as using these properties and relying on them are problematic (not least for the point mentioned by Neils himself). Make your own, dedicated properties that are aptly aliased for their task.