1- Cut the tree at the base, keep the upper part of tree alive by putting the base in water during the process.
2- Make a 3D scan of the upper part of the tree.
3- Use that to make a 3D print of the new clear, illuminated structure that will replace the heartwood, the dead core of a tree that we
get wood from.
4- In segments, remove the sapwood (the living layer between the heartwood and the bark) from the living tree and transplant it bit by bit to the same physical location on the 3D printed analog tree.
5- To assure the xylem, the system that brings water and nutrients from the roots, continues to function, small recessions in the plastic part would be left to pump supplemental water and nutrients to the phloem, the living part of the bark that makes up the living tree, feeds the leaves, provides growth etc. Could trim it bonsai style to keep everything lit, but presumably could just add new lit cores to new branches as needed.
This little illuminated forest might be something you'd put in a sculpture park by an art museum or maybe a memorial park. Just having one of these at the top of a hill might be pretty.
Of course you wouldn't be limited to the tree's original shape. You could make new artistic tree sculptures any shape you wanted.
Inspired by these fake, plastic birch tree trunks with a light in the middle that seem kind of pointless. This Frankenstein approach would create a living glowing tree with leaves, new growth, everything except the not glowing part.
ADDENDUM: Could just use hundreds of lines of LED lights inserted like catheters but you'd need a pretty sophisticated 3D drilling / guidance system. I'd also to a prototype desktop bonsai tree first. Also wonder if you watered them with UV light glowing liquid and shined a blacklight on them if the leaves would glow too.