Is it a good idea to put some rotted manure in the holes I plant my brussels, cabbages and caulis in?
No, they want hard firm land and a little lime.
an inch chunk of rhubarb is said to help
I don,t think it is worth it for the amount of manure you would get in i would dig back putting manure in each trench and putting the soil from the next trench on top of the manure.
Failing that you could put a ring of Growmore and Fish Bood and Bone around each plant and let the weather wash it down to the roots, just ensure that the Growmore is not touching the plant stem.