Strictly speaking, some form of polymorph effect would allow you to qualify for a prestige class, and you only need to qualify in order to take the first level, so if you manage to level up with such an effect, you could take that level and then ignore it from then on.
Not many DMs will let you do that, including me: I would much rather waive the racial prerequisite on the class than set the precedent of allowing such shenanigans. But RAW, it would be one approach for prestige classes. It does not help for feats, since you need to meet the requirements for those to use them as well as to select them.
If the race required is one of the “races of stone,” that is dwarf, gnome, or goliath, the Stoneblessed prestige class from Races of Stone allows you to count as one of those races as its capstone. The class is pretty easy to enter by 3rd level, and only three levels long, so you can have the class finished by 5th and thus be eligible to take the race-locked prestige class starting at 6th, assuming you meet the other requirements. Unfortunately, its other class features are pretty mediocre, so you would have to be in a really specific situation where it’s better than either just being the required race or skipping the prestige class.
Stoneblessed also includes an adaptation section, suggesting that it could be adapted for use to qualify for any race, just by creating class features for that racial choice matching those availble for those looking to count as dwarves, gnomes, or goliaths. It even gives tips on the kinds of things these should be.
But beyond that, you are out of luck: I am not aware of any similar options in all of 3.5. Even the similar three-level elf-friend prestige class in Races of the Wild, the ruathar, does not have that same feature for elves.
