I am not sure if I understand your question right. To rephrase it. I think you would want to have the repo local copy updated on a regular basis. However you would want the working copy pristine so that the resulting build is a clean. Considering this is your question below is what I would suggest.
To my knowledge svn export might not be the be best option for this. Because the purpose of svn export is to obtain a unversioned working copy of the svn repo. As it is unversioned, svn client would not really know from where it has to start the update.
The best option i can think of is this. Checkout the copy of the repo (local copy, LC) in a location. This LC should be updated during the build process. Make a copy of the LC in a different location and use it for performing the build. Below are the commands you would require
1. svn update <arbitrary path>(in the working copy)
2. copy <arbitrary path> <build path>
3. find <build path> -type 'd' -name '.svn' (if you would like to remove the .svn hidden files, but they are not going to really hurt the build process)
Some Options for Eliminating the copy time from factoring in the build process time
If you would like to save the copy time during the build process probably you can do this copy operation after each build and svn update the copy just before building (assume the .svn folders are retained).
On linux two folders can be kept in sync using rsync. The build copy can be made to reflect the updates in the pristine copy.
In Windows, there are a few tools to achieve sync suggested above. I have not used them but I will provide you the links to try it yourself.
http://lifehacker.com/326199/synchronize-folders-with-synctoy-20
http://www.techsupportalert.com/best-free-folder-synchronization-utility.htm