I did it before on E-Smith 4.1.2, there was a kernel patch that would allow you to load balance across two external interfaces. Obviously there was no GUI for it, and /sbin/e-smith/console still only saw one external interface, but using the command-line after recomiling my kernel I was able to NAT load balance across a cable modem and a DSL line.
Additionally, you can also load balance across "virtual" or "aliased" devices as well. I'll have to see if I can dig up the patches, hopefully I stilll have that bookmark, this was at least one year ago.
Thanks,
Nathan