Solidity – Is ‘tx.origin’ Always an Externally Owned Account (EOA)

MetaMask accounts are EOA accounts. They are not seen in Geth because the keys have not been imported from MetaMask.

Basically, MetaMask creates a hierarchical deterministic wallet hd-wallets from the seed (words). An HD wallet has many keys and each of these are EOA. (The MetaMask UI only shows some of them, but more keys can be continuously derived.)

In Geth, you don't see the MetaMask accounts because you would have to import the private keys from MetaMask to Geth.

DELEGATECALL is an instruction interpreted by EVM. So to execute DELEGATECALL you have to have a program (contract) that gets executed on EVM.

DELEGATECALL takes six operands, one of them is the address of the calee. So it can only call deployed contracts. Note that so called library is just a special contract which operations are called with DELEGATECALL.

So to answer your question directly as I understand it: No - it is not possible to execute arbitrary code from an external account.

What you can do though is you can create a contract that implements the logic as you want it and uses DELEGATECALL to preserve sender and value when calling other contracts.