If I understand your question correctly: it looks like you want to demux the incoming DS3 into 28 T1s, run the T1s for 300 feet over your existing 100 pair, and then mux them back into a single DS3.

I looked at a similar situation a while back and while we ended up going a different route, here is what I found. Please keep in mind that I never actually attempted this, so treat this info as a starting point for further research.

I believe a Carrier Access Wide Bank 28 can do what you are looking for. If you are willing to risk eBay, they can now be found for under $400 each with the necessary cards installed.

That being said I can think of a few reasons you might not want to do this: First of all, for your price range you are stuck with used equipment. That means no tech support from the manufacturer, troubleshooting without knowing for sure that you actually have working equipment, and the possibility of getting equipment with the configuration locked behind an unknown password.

Second, I don't know how much success you are going to have running 28 T1s together on a single 100 pair. There are a lot of experienced T1 guys on this board, I'd gather some feedback from them before trying this.

Good luck and I hope this info helps!