if you know how to search and replace in a text file, you could create two directories, one with the images you will put in your final document and one with a low res version. you can then open the .sla file and search and replace all the images and change the path to match the version you want to work with.
it should be easy to create a script to do this.
otherwise, the solution is to split the .sla by chapter and merge them at the end or merge the resulting PDFs.
ciao
a.l.e