Hi Vortex. I too love the wingtip trails, and I've got a good feeling that you might be in luck here because I am am pretty sure that the wingtip effects don't rely on the fx_fueldump.fx file, since I am still getting them even with the file renamed.
But I think the news is even better than that because, as best I can see, the fx_fueldump.fx file only made its first appearance on the Acceleration Pack disc, which means that the only aircraft that are likely to use it are the Acceleration ones (the F18, the P51 and the chopper) and not any of the original FSX aircraft. Now, fuel dumping has been available from the outset of FSX by using CTRL+SHIFT+D so I tried to confirm my theory by restoring my .fx files back to normal then loading up the A321 and having a look at her while she was dumping fuel. Tada...no visible fuel trail, even though the fuel gauge was dropping like a stone. (I also tried it on the P51 - same result)
Now, admittedly that doesn't constitute exhaustive testing so I can't be 100% sure, but I have a very strong feeling that renaming the fx_fueldump.fx file will not affect anything other than just the fueldump effect on the F18 (and probably no other aircraft at this time). Do you fancy giving it another try, chief, and let us know how you get on?