|100% pure kitty here, no human DNA required.|
Still, that is enough to warrant serious ethical consideration. I cannot speak for all Humanists, whose opinions on such a complex matter will no doubt vary considerably. But I can speak for myself, and talk about why my position springs from my own Humanist values.
First, I begin with a very broad fundamental principle. In this case, the idea of what is 'special' about being human. Having no belief in souls or the like, one would think I'd be left with nothing but atoms, DNA, and so on. Thus, one might imagine my definition of 'human' to be the biological species called Homo sapiens. Not so.
Surely, the thing that makes human beings special is not that we have two arms, two legs, and so on. Our genes and our very bodies and brains are simply matter - not of much significance. Rather, even the naturalist must acknowledge the significance and existence of the metaphysical. By that, I don't mean supernatural, but rather those things which are not merely physical objects. In this case, we're talking about patterns, and functions, and emergent properties of those objects in accordance with how they interrelate to one another - and the appropriate value judgments we place on those properties for our own well being as a people.
The human mind is a process. It is a process which allows for the conception of ideas, memories, the capacity to make choices, and the capacity to imagine and form high level concepts. Importantly, it also shares with lesser minds the capacity to experience the first-person sensation of suffering. While this process emerges thanks to the activity of particles, it is more than merely the atoms that make up the brain; it is that process and the capacities of that process which are of significance and of moral concern. More than atoms, proteins, DNA, tissues, or the human animal; the human mind - is - the human being.
It is with this perspective that I am not opposed to first trimester abortions. There is simply not a human mind present, and certainly not before the brain activity that allows the process of mind. Consistently, I am also not opposed to 'unplugging' a body being kept alive by machinery which has no brain. Also, consistently, were we to ever discover an extra-terrestrial non-human intelligence, I would consider it more kin to us than to the other creatures with whom we share this planet and biological relation. It is with this moral and ethical focus on the "person" over the biology, that I approach the issue of human/animal hybrid research.
Proceeding thus, I can say that I have no problem with anything that is created, even a Moreau-like monstrosity, provided it is destroyed before complex brain activity develops. Before a 'person' has emerged, it is no more significant than any other clump of mindless tissue. If such a thing produces helpful knowledge that will cure the sick or heal the injured, then so be it - provided strict and enforced precautions are taken that it not be developed.
And, what of creatures that are developed fully? Here we must be careful that the type of hybridization is not philosophically or morally significant with respect to personhood. For example, should a mouse have a liver that contains some human liver cells, this should be of little concern. Or, conversely, should a human have a pig's heart this too is simply a matter of equipment and machinery.
But, it is important to note that there are still many things which we do not understand about the working of genetics. What specific genes allow for the development of a nervous system with human-level awareness? Could it be that one important ingredient for that is hiding out in an area of the DNA sequence we currently believe handles only the metabolism or the liver? We must be extremely cautious here, or else we might conceivably create some animal which has human-level awareness, or something disturbingly close to it. Far from some cartoonish talking gopher, we might not be able to communicate with such a creature well enough to even realize what we had created. If the gene mixtures begin to affect the cognition of the creature, we begin to enter serious ethical territory in the creation and treatment of such a being. Like the ETs I wrote of, this sort of creation would be our kin in ways more significant than its physical appearance, and experimentation on such a being would be unthinkable.
So, when it comes to those creatures brought to full term, I would advise very strict oversight and enforcement and proceeding with extreme caution. We must also make certain that such oversight bodies are not tainted with profit-motive or influence from profit-seeking entities. We should also seek to instill into any current accepted policies against human experimentation, provisions which account for 'human related cognition' in the definitions used.