Could metal hydride storage tanks for hydrogen store helium as well, given their repective atomic radii?
Hydrogen does not just insert itself physically into the metal.  Although only a few metals form stoichiometric hydrides with predominantly ionic character, most of them (and all those involved in hydrogen/hydride storage) form some type of bond, generally involving saturation of the hydrogen 1s orbital with ionic or polar covalent bonding.  Helium simply does not have such capability for the same reasons it does not bond with much else, and it is generally not significantly soluble in metals.

An introduction to various methods to store hydrogen is given by Wikipedia.  The 
US Department of Energy reviews some proposed materials here.