When a disciple traces line of sight for his radiant light skill while having elm greatbow equipped , do adjacent figures block his line of sight? In other words can weapon passive abilities be used in something other than attacking with it?

I was going to say probably not, since most weapon abilities affect attacks with the weapon (see Bloody Dagger/Dual Strike interaction) ... but a much later expansion (The Chains that Rust) includes the Bone Wand and Fists of Iron, both of which provide buffs that are always active, which the Elm Greatbow wording is consistent with..

I say yes, but someone else can think differently.

Edit: a better example would be Bloody Dagger/bare hands interaction. Bare hands doesn't get Bloody Dagger's buff, based on an FFG answer posted by Zaltrye https://community.fantasyflightgames.com/topic/189547-skirmisher-dual-strike-bloody-dagger/?do=findComment&comment=1835108