Feel free to refer directly to the yellow paper or other similar baseline documents - I've been reading those documents looking for an answer to my question. I wish there were a "language-lawyer" tag similar to the one on stackoverflow, because I'll only accept an answer that proves conclusively that the answer is "yes", "no", or "unspecified" - I won't accept an answer appealing to your gut feeling.
Solidity contracts appear to operate at a significantly higher level of abstraction than the EVM itself.
For instance, to determine which contract method a transaction executes, the ethereum wiki states:
The first four bytes of the call data for a function call specifies the function to be called.
Going back to the yellow paper to figure out what happens during a message call:
The account’s associated code (identified as the fragment whose Keccak hash is
σ[c]_c) is executed according to the execution model.
Now, the associated code is simply the output of the contract creation code. The yellow paper also states:
...the resultant byte sequence from the execution of the initialisation code, specifies the final body code for the newly-created account
So in the yellow paper there is no mention of contracts even having methods, much less of the first 4 bytes of the transaction data specifying which one is to be called. I gather, therefore, that it is Solidity (or another layer above the EVM itself) that outputs contract EVM bytecode that branches based on the first 4 bytes, and this is not a feature of the EVM.
Therefore, this question is probably more related to Solidity than contract execution, and may even have a different answer case-by-case for different smart contracts that are implemented.
That being said, the question is this:
Take a look at this transaction. I created a well-formed call to a smart contract, sending 1 token to an address, however onto the transaction data I concatenated the hex data 0xdeadbeef. I would like to use this as a way to attach application-specific memos.
Given that this works today, is there any reason why it wouldn't work in the future?