Looks like I came through for you!
Here's the equipment I was thinking of.
This is a PDF spec sheet on Paradyne's etherloop series of bridges.
https://www.paradyne.com/pdf_central/etherloop.pdf That product should do exactly what you need it to, and they're not too pricey.
Another option, that will do nearly the same thing -- for way cheaper, but I can't find any 100% documentation on, is Efficient/Flowpoint's Speedstream 5250 -- here's an ebay search for them, they're about 15 bucks a piece:
https://search.ebay.com/search/search.dll?cgiurl=http%3A%2F%2Fcgi.ebay.com%2Fws%2F&krd=1&from=R8&MfcISAPICommand=GetResult&ht=1&SortProperty=MetaEndSort&query=SpeedStream+5250+
They supposedly will auto configure to the highest speed possible considering line conditions. That could be something worth trying if they actually work.
I found a slashdot article that chronicles basically the same problem:
https://ask.slashdot.org/askslashdot/03/01/30/1739238.shtml?tid=137 I think you'll find a solution within those ideas.