It may be impossible as well. The signal would need to be in a format that the player would accept. You may be better off just getting a newer player that does what you want. Could be cheaper and eliminate the hassle of soldering connections that may or may not work.

*Firstly, to answer your question*
Far far to many variables to make a definitive call off the information you've provided. But It all depends on the model really. I'd say 40/60 chances you could do it *AT ALL* with any random unit. And 20/80 chances that it would be easy. Some electronics come in a few slightly different versions that both use the same board, just with the jacks missing and other parts and what have you. If your unit was like this then it should not be all that hard to do. But I just can't say for sure yes or no with the information you have told us.

There is also always taking the screen it's self off the DVD player and building a standalone circuit to drive just that part. This has better chances of being doable... period and gives you the best options for inputs, because you have full control. But that's really high level complex circuit bending/building and the costs are also steep. Only for the pros to do in reality.

*Now, to scrutinize your question*
I'd say don't bother, sell what you have on ebay, save up some more cash, then just get your self a better unit. Or buy a small LCD TV and build or buy a power supply for it to make it portable. They sell LCD TV's that are already portable. Like for in car DVD players. This is what I would do, and this is coming from someone who knows and loves to do this kind of stuff and does it with with out needing to ask others for help. I'm also broke as a joke, so you know it's in your best financial interest to do it this way too.