Hi,
I'm afraid the bandwidth throttling feature you are looking for is not available when imaging. When you have WAN setups, you should really be looking at package servers which act as local caches for package and image stores.
Some organisations also create a scratch drive on these package servers to provide a little extra flexibility for local imaging. This allows a fast image upload scenario locally, and should that image be deemed suitable for uploading to the central store, this can be performed using RoboCopy which has in-built throttling.
For that to work smoothly, the PXE boot images will need to map a drive to the scratch location for the purposes of these ad-hoc uploads.
Here's some RoboCopy Links:
http://ipggi.wordpress.com/2008/01/17/robocopy-basics/
http://technet.microsoft.com/en-us/library/cc733145(WS.10).aspx
http://www.minasi.com/forum/topic.asp?TOPIC_ID=9173
Kind Regards,
Ian./