The epoxy barrier might work, but in general encapsulated metals are risky because they are impossible to inspect for corrosion and cracking so fail without warning, and the encapsulation can block surface oxide formation which causes crevice corrosion- especially if small amounts of salt and water get in there, which they will over time, even in epoxy.

I’m sure what you are saying could be done- especially to basically add stiffness to key regions of a carbon racing bicycle, but it would be experimental and I would not trust it to last a long time

Yes, that is how carbon fiber racing sailboats are made

> Magnesium, especially a casting, can be something like an order of magnitude cheaper.

That doesn't seem to be borne out by bike prices - it's entirely possible to buy a very usable carbon fibre bike for approx £1000 but I can't recall seeing a magnesium framed bike for £100.

Edit: looking at cheap frames on AliExpress, you're not too far off. I saw a magnesium alloy frame for approx £80 and a carbon fibre frame for £350. Not quite an order of magnitude though.