I believe the cable to which you are referring is for Onkyo's RI (Remote Interactive) system and you are correct that it will only work with Onkyo components. It allows you to daisy-chain components so that if you point the dvd remote at an Onkyo receiver, it will send the command on to all devicdes connected via RI and if one of them is an Onkyo DVD player, it will accept the command. It allows other convenient features as well, such as dimming the receiver causes the dvd player to dim as well and turning on the dvd player first will cause the receiver to turn on and select the dvd input. Other brands use similar schemes: Sony (S-Link) and JVC (Compu-Link) to name a few.
Yes, there are IR extenders/repeaters available. Radio Shack sells some, Prism makes some, and Crutchfield.com carries a few as well. An IR extender is one option but it requires that you place one transmitter in line of sight with the component which may not work too well for you if the dvd player is in a location that makes it difficult for the IR receiver to 'see' the signal from the remote.
There are other alternatives that are better and it just depends on how much you are willing to spend.
1. Get a universal remote with a more powerful emitter. Home Theater Master remotes are what I use and it will work from over 50 feet away in my house. I've read that Harmony remotes also use more powerful emitters. Besides a stronger signal, you get the benefit of a more advanced remote that can easily be programmed to control ALL of your equipment.
2. Use a remote that has RF capabilities. Again I like the HTM remotes. You could get an HTM MX-350 (Osiris) and its RF base station for about $200. The RF receiver can be configured a number of ways. You can place it such that it is facing the components and turn on the front blaster or you can place it anywhere convenient and run emitters from its back panel. The emitters need to be placed near the components (either attached directly over the IR eye on the component or just within the rack so that the component can see the IR from the base station).
Note that there are cheaper, although less reliable remotes with RF capability as well, such as the One For All URC-9910.