A kettle element is cheap, and it will work in a fermenter as a boiler.
BUT
It is a homemade electricity vs water solution, i personally think that it will make for about the shittiest BIAB rig you could build yourself, and fermenters are expensive and smaller than you really want for BIAB anyway.
I agree with Dan 100% - i think that an over the side immersion element is about the best bit of kit you can buy, no matter what sort of brewing you decide to do, will turn ANY vessel into a brewing vessel, and if its big enough and heat proof, into a boiler.
My ideal BIAB rig for single batches is a 50L pot and an over the side element. If you can find a nice food grade 50L plastic container that can take the heat... Then I would see no difference between it and any other pot.