...if your aim is purely to reduce network load on your central SEPM, you've actually got quite a number of options.
Option 1 - GUPs
These are SEP clients that act as deinition proxies. If configured correctly, you could get all the SEP Clients on a remote site to talk to a GUP for definitions, the GUP would then be the only one to download from your central SEPM. Because the GUP caches the definitions locally it should only need to downdload each definition request once (therefore reducing the load on the SEPM).
More info on GUPs can be found here: http://www.symantec.com/docs/TECH191394
Option 2 - Additional SEPM Sites
This is only viable if your remote sites have a lot of SEP Clients on them, and if the distributed management would also be of benefit (note Symantec Best Practices recommend keeping the number of SEP Sites to a minimum). Doing it this way essentially means each site has its own SEPM, and the SEP Clients download definitions from their local SEPM.
Option 3 - LUA
The LiveUpdate Administrator software allows you to create your own LiveUpdate server, and schedule to pushing of definitions to servers closer to your clients. This wouuld provide the additional benefit of being able to schedule when updates are tranferred across the network to your remote sites.
More info here: http://www.symantec.com/docs/TECH93409
Option 4 - Symantec LiveUpdate
This is only applicable if the remote sites are fairly small and have their own link to the Internet (i.e. they don't go through your main site for Internet access). You could use this option to tell the SEP Clients in your remote sites to go straight to Symantec for definitons.
Each of the options have their pros and cons, but should give you an idea of what you can do.