They don't have to be mainly assembly tools.
Make a skeleton part with surfaces and planes representing where you want your geometry to be (like you would with a solid in a standard envelope).
Now take that part and insert it into a part before you start modeling.
Now use the imported surfaces to guide the creation of the part.
Then when in the assembly pull in the skeleton part and mate the skeletons in the previous parts to that.
This was presented at SWW for building cabins in boats. They built a master layout from surfaces that represented the boat hull and the fore aft location of the cabin walls. Then each cabin could be modeled based on this master layout including trimming the cabin wallls to the hull without needing in context relations or having to work in the large assembly.