A nervous system doesn’t necessarily imply sentience. Someone who is brain dead but on life support has a nervous system, yet no activity will be registered in the brain no matter what kind of stimulus happens to the body. One might bicker about it being impossible to know whether a brain dead person has any kind of qualia, but I’m pretty doubtful and think it’s reasonable to assume they don’t based on the evidence available. Hell, while we’re on the topic let’s just shift our hypothetical a little bit.

What if someone volunteered to donate their body to science after their death OR brain death. A terrible tragedy happens, and they do indeed wind up brain dead. The will is solid and a research institution (for the sake of argument, we will stipulate a morally sound institution) wants to keep that body on life support and use it for medical testing purposes to avoid having to test on animals or fully conscious and sentient humans. Assuming the person made such a donation in good health and a sound state of mind, I don’t see anything wrong with this. The family might object based on sentimental attachments to the body and the person the body used to be, but without a functional brain I don’t really think we’re talking about a “person” anymore, not morally speaking. Much closer to a corpse than a person in my opinion.

Having a human body grown without consciousness, sentience, or a social identity/connection at all overcomes any kind of remaining sentimentalist objections. If we accept that someone can donate their body and organs to science or other people upon their death/brain death, I think it follows that we can permit non-conscious and non-sentient bodies in general to be used for such purposes. Again, as long as the body is grown expressly without those things, I don’t see a coherent argument against it.