Add CCK form with overlay or colorbox
If I only display the node in colorbox
  (which is installed and used for other
  features) then I miss out on the
  multistep block.


A "quick-and-dirty" solution that I can think of, is to embed the multistep block in the node using the Insert Block module. Notice, that if you are using D7, you can check this, which is a community submitted port of the module. 

If this is not what you need, please add a couple of screenshots of the form and the multistep block so that we can understand exactly what you are trying to do here.