The resonant concept does interest me, but I believe I might prefer taking a different approach to reach a similar end. Provided it doesn't break lore (which, to my knowledge, it shouldn't), the idea I'm mulling over after re-reading the lore books and discussing things in this thread would combine selective breeding (i.e. using bloodlines known to have produced aether-capable purebloods at some point), the use of a surrogate womb (implanting a fertilized Garlean egg in the body of a servant believed to possess the echo, for example), and intentionally exposing the unborn child to gradually increasing densities of aether over the course of its development. One could even involve Zodiark in the process if it was a project signed off on by Emperor Solus. No doubt he has access to the dormant Zodiark's energies, meaning he could imbue the children with a tiny portion of that power in an effort to increase their odds of having the echo.
The end result would be a modified Garlean possessed of aether manipulating abilities and access to a more natural form of the echo - one which requires significant amounts of time, training, and experience to use effectively - rather than resonance, which appears to yield an immense increase in prowess up front. Would definitely be interesting to see how that growth played out. After all, such an individual would not be able to receive that sort of instruction in Garlemald, and no pureblood wanting to maintain even an iota of social standing would go asking conscripts for advice in the first place. It's not likely they would be able to advance these abilities much, if at all, before leaving the Empire.
This new angle has also had me rethink the character's possible reasons for leaving Garlemald behind. Rather than doing so for survival, perhaps it was for the sake of reaching new heights? I can't picture an individual that would have been constantly pushed and prodded to meet or exceed the highest standards of excellence within the Empire for essentially their entire life being content merely to live out their life in servitude, never attempting to explore the gifts they've had since birth. I don't see the Garleans being foolish enough to tell an individual created through this process of their true nature, but you can bet such a person would always sense something was off. That sense of something missing would inevitably give way to the desire for something more - acknowledgment of dreams unfulfilled, potential untapped. That, in my opinion, would be enough to motivate just about anyone with the strength to do so to strike out on their own in search of themselves.
If anyone has any reason this would violate lore, please let me know. If not, I suppose that just leaves me with one final decision to make: which in-game race to best represent the character with. Au Ra and Elezen both have the correct height ranges and builds, but the former has all those pseudo-draconic/demonic/whatever traits while the latter looks awful in most helmets and has the neck of a giraffe. I suppose that leaves Midlander or Highlander.