I haven't gotten to actually try this but maybe someone else did.
Pretty sure I saw a mage diana last night and it looked weird how the spell triggered, it was one shield right after the other... Seemed a bit wonky, as far as mundo assassin goes I haven't tried it but I'm guessing the berserker buff would take precedence... It would be funny though if it did some kind of double jump, jumped to front line and jumped to furthest away... Really depends on the time element I think, didn't they add a pause to the assassins jump time during set 1? Like .15 a second.... I think by default that would make the berserker jump happen first because its almost instant. Although its likely they just copy and pasted the code while changing targeting variables... Now i'm inclined to try it and put a jeweled gauntlet on him too so he can get them juicy crits.
As i recently had Renekton with the assassin item i can tell you the item overrides the berserker buff and will jump to the furthest target. Cant speak for anything else though.
Like dudekri said; the assassin buff overrides the berserker buff. I accidentally put it on my olaf once (when trying to make a 
instead) and he jumped to the back
And also If you put a ligt item on malz the little pets are also considered light?
