Do you use IPLOT? This could solve the problem as you can set up an IPLOT file to plot partial drawings from various fence locations.
On the other hand, if you need to break up the model, I'll use levels/layers as a for instance; say you need to separate the civil from the mechanical layers and put them in two separate files, you should be able to write an MDL/VB routine that will do it. Describe how you're trying to break up the file and we'll go from there.