Hey, I just sculpted a nice model, using the multires modifiert.
(lowest resolution is a simple subdivided plane, 4th resolution is some kind of high poly sculpted plane ;))
Anyway, the multires modifier has a “pack external” option.
This allows me to export (whatever??) to a btx - file.
following questiond:
What the hell is a btx file?
How can I use a btx file?
What’s it good for?
I used a 2.53 multires build from graphicall.
Hope u can help me (once again these days) 
http://www.blender.org/development/release-logs/blender-250/sculpt-and-multiresolution/
External Displacements File
Multiresolution displacements can now be stored outside of the .blend file. This helps keep the .blend file small, and makes it possible to load the displacements in memory only when needed.
This can be considered much like a displacement texture map that only applies to a particular mesh, and that you might store as boots_disp.btx next to a boots_color.png, for example. Saving to this file happens automatically when saving the .blend file. For performance, you can ensure that it does not get loaded into memory when opening the .blend file, by setting the viewport level to 0 in the multires modifier.