Just a quick comment. I have worked extensively with DS3's on 7200 platforms. In that case there were two types of cards channelized (very expensive) and Clear Channel Cards. If your Routers will support a Channelized card then there should be no problem accomplishing what you are trying to do.
Also I have a client who is terminating a DS3 to an ADTRAN unit then using CSU/DSU's to break out the channels. If for some reason your router cannot handle a Channelized Card you might have some options using other termination methods then handing the links back to you router after the L2 segmentation has happened. I also have another client that is moving to Metro fiber with the local cable company. In this case we are terminating 10 Mbps circuits at all WAN sites a 1Gbps like to the CORE site and each site has a full T1 Side channel that is being converted to analog for phones. I'm sure you local carrier could also configure something like this if for some reason DS3 termination is not an option.
