Okay, but then this is in fact an alternative to just having to trust people. In cases where the stakes are low, trust can be extended without any additional evidence. If there is cause to believe that stakes are high, then you don't necessarily have to rely upon a declaration alone.

(Regarding phantom limb syndrome, I agree it's not fully parallel. However, body identity integrity dysphoria is not easily treated, so I don't think your answer works there--you can use antidepressants to "treat" people with gender dysphoria too! Furthermore, trans people do not end up with a complete set of "fully functioning body parts" if they undergo physical transition. Some things work decently; others only kinda; others not at all. In particular, reproductive capacity with new body parts is not maintained.)