I every now and then use blackstrap molasses (it's called black treacle here in the UK) during flowering but I wouldn't use it to do a flush. I only use it to feed the microherd in the soil, and I use a terra preta style soil which I reuse over and over (I've been using the same soil for a few years now).
Muscavado will work for feeding the microherd but its not anywhere as rich in minerals etc as molasses is.