Can a bike frame be retrofitted with internal cable routing without Disassembly? Would I be able to have a local framebuilder retrofit an existing steel mountain bike frame to install internally routed cable guides on the top tube without disassembling the frame? I would probably prefer to install lugged guides for this.

Why? Certainly not for performance reasons. I just like internal routing, which I have on a couple different bikes already. Less external stuff to get caught on while riding and easier to accommodate frame packs for bikepacking.
No. If nothing else you'd need to strip the frame to have it repainted after the guides are brazed into place.