Re: the original question....in my state, the list of "RN only" things is fairly small, and in others it's very large, so we may be doing an apples and oranges thing. In my state, I can't declare a death with another nurse like an RN, I can't hang IV chemotherapy (but I give PO all the time), I can't initiate a care plan, if I do the initial assessment, an RN has to validate it (which usually consists of someone yelling, "Nerd, you done yet? Good, you're validated.") and an RN has to do most of the "official signatures" for things like restraints. In my facility, I can spike and hang blood because of extra training I've had. Some states, LPNs can't touch it. In most other things, we function identically, so I could be a learning resource to a new RN for specific things -- and have been.
What I would suggest is go for the smartest, best teacher in any circumstance. We have a LPN who's also an EMT, and she can hit a vein that nobody else on the floor can even palpate -- I learned more about starting IVs from her than I did my RN preceptor. When I finish RN school and precept, she's who I'll get to teach the newhires how to stick somebody. Another LPN is also a respiratory tech -- she taught me more about ABGs than I've learned in either LPN or RN school, and I've used what she taught me to help my struggling RN student classmates. The RN I learned the most from was an old ICU nurse, and she dragged me to the edge of my license and taught me how to live there, and there are folks alive today because of the things she taught me. She fought to get my ability to access a portacath blessed, and there have been nights on our floor where I'm the only nurse, regardless of license, who's ever accessed one, so who does it? That's the difference between a born teacher and a teacher in it for the money alone (2dollar/hr differential at my hospital).
Yes, there are things that only RNs should do. But rather than the current one preceptor/one preceptee thing we have at our facility, I think a more cafeteria approach would be beneficial to the newhires. If you've got an LPN on your floor who can find a vein in anybody, vs. an RN who can't hit a subway tunnel, who would it be best for you as a nurse to learn that particular skill from?