It seems that you all have VERY unrefined taste buds! I LOVE marmite and have managed to find places in the USA to buy it!
It is very salty tasting but it is only yeast in it so I don't think it would be a problem for the chickens especially if you spread it real thin like you would if you were going to be eating the toast yourself. A jar of Marmite lasts me months and I have some nearly every day... Give it a try and see what your chooks say. Let us know too, I don't know if I could bring myself to share something like that with the chickens!