Sword mod for cleric flail please?

It'll work, I've seen people use NPCs as weapons, the only issue is the animations will likely be...off. if the flail is a 1 handed item (which if i recall it is), and the sword is a two hander (may or may not be on the clerics because of the shield) they will use what ever the current weapons animations are.
you also may or may not see some stretching of the sword when you attack (i've noticed a stretching effect of the flail, it may or may not do the same)

I have had success modding over items that are not the same "type"
the image below shows the scythe from the basin of the fallen end boss modded over my level 24 blue staff.
yes they are both a "staff" type but the principal behind it, and even the NPC modding are the same.

Damn, that scythe is awesome.

Closing. No begging please. If you need something made try PMing.