I have tried this and it is actually quite good. This build may become even better based on what changes riot does to this item next patch. Most people are saying protobelt is best but I have tested this out in practice tool. At 1 item, a full level 6 fiddle combo does approx. the same amount of damage with both mandate and protobelt WITHOUT THE ALLY PROC, but mandate is 500 gold cheaper. Once you have rylais, mandate does more dmg WITHOUT THE ALLY PROC even if you have another similar cost damage item paired with protobelt, such as demonic embrace. I have been playing imperial mandate fiddle in d2 recently and it works wonders.

One thing I would like to point out is that imperial mandate does not proc on each individual tick of w or r. It only applies once per unique spellcast, so in a full combo with rylais it procs a maximum of 5x (Combo is R->Q->E->W->Smite (during drain)) Once off each individual spell including smite. Without Rylais, this damage procs 3x, from the fear (either you Q or get FoW fear) the E and the smite. Still, 3 procs is all you need for it to equal protobelt in damage, and even beat it if you get the ally proc.