I solved it in a similar way as pekayatt. By adding the missing button names to the InputCoreTypes class. This allows access to the tracker buttons from Blueprint as Generic Steam Trigger, -Touch -Grip or -Menu. Basically how they were named by Epic in the SteamVRControllerKeyNames namespace.
I have added a patch (as .txt) to this post (I cannot link to my Github, as UE is a fork from a private repo). Pull request for those interested: https://github.com/EpicGames/UnrealEngine/pull/4450
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