In other words, you want a quick way to change any reference to a built-in sprite to references to a custom sprite. It could definitely be done, and I could write up a Python program to automate the process for you in less than five minutes. Integrating it into the editor is... more complicated.
There are two general approaches here: one is to change what the references refers to, i.e., make it possible to give AAO the command "whenever you see character X and built-in sprite Y, it actually means you pull these special images." I would rather avoid that option, since it creates an additional test the trial player it tries to load a sprite, and it makes the trial data harder for a human to read. If the trial data was ever read by a human, it would also make the data harder to read; now you have to check if a builtin sprite is swapped to a custom one. My deepest concern, though, is that it muddles the trial data structure. v6 has it so "A negative numbered sprite means it's built-in." A change in this direction would turn that rule into "A negative numbered sprite means it's built-in by default, unless there's some global override affecting this particular sprite." It's such unnecessary complexity!
The saner solution is conceptually elegant, but could be a pain to implement. You'd need some interface to select two sprites, and swap all references to one with references to the other. The code that does the replacement would be two clean for-loops, one over the frames, and one over something Unas is building called frame-presets. But the editor interface is much trickier to design. My instinct is to create a new button on each profile, and borrow the "animation select" code from frame-building to select the animations.
...There's also the matter of how soon this could be designed. I don't feel comfortable building this until I've read over all the relevant code for the profiles tab and the animation-select mechanism, which is not a high priority for me at the moment, due to real-life obligations, and even then, I want to focus on the Glitch Hunt when I have time for AAO Open-Source. (Which I won't really focus on until after the Platinum rewrite, which comes after Catalysis...)
If there's a strong demand for this feature, I could write it up, but until then, I think you're better off sending me a PM with the substitution you want to do. I can do this easily, when I don't have a GUI implementation to worry about.
[D]isordered speech is not so much injury to the lips that give it forth, as to the disproportion and incoherence of things in themselves, so negligently expressed. ~ Ben Jonson
Current AAO Development Priority: Issue #94: Grayscale Mode