I don't think anything like that exists in the commercial realm because such problems are usually solved by inserting a switch in between the devices - thus no commercial viability. Why don't you do that and see what happens?

I've got an Ethernet hub at the office that most Cisco routers work fine in, but plug in a Cisco 804 and it starts shrieking about "late collisions" Plug in a Westell DSL modem and if the Westell has early-revision firmware it won't work at all, flash update the Westell and it starts working.

I have this hub on the test bench as it occasionally comes in handy to have an oddball ethernet device around.

I also have the same problem with a router/PC running an Intel Etherexpress Pro 100 plugged into a horribly expensive switch at a colo site. The colo vendor normally hard-codes the autonegotiation on this switch to 100/Full, because they say that autonegotiation is unreliable. My gear loses about 2% of the packets when they do this no matter how I have my side set. When they set to autonegotiation on their side (after lecturing me on how doing this is a Bad Thing) and I set to autonegotiation, the packet loss goes away. However the downside is about 1 in 10 reboots, their port goes to a funny state and stops talking. I can kick it by a quick set of the port on my side to 10base/half, then back to auto.

And I personally made them let me go through their cable plant and cut off and recrimp and repunch every jack and plug in the connection between my router and their switch (and I won't even go into the mistakes and messes I found while doing that)

At one remote site we had a customer with a Cisco 1605 plugged into a fiber module that is hard-coded 10/half. The error log in the router was stuffed with late collision errors. I went to Goodwill, bought an old unicom 4 port 10Base T hub for $4.99, found an A/C adapter in a bin there for a buck rated the right power and soldered on the correct plug to the end, inserted the hub in between the fiber module and the router - voila, problem vanished.

You are way overengineering this. Accept the fact that Ethernet isn't a particularly good kind of interconnect to use in between network devices and the reason it's used is because it's cheap, buy a switch, and move on.

I would also venture a guess that your a young guy. Old timers that dealt with Ethernet years ago had these problems all of the time and have many stories to tell. You haven't lived until you've debugged a thinnet network.

Ted