Thanks. Now, it is cryptographically possible (for future versions perhaps, let us disregard whether or not it would ever be pulled into core) to sign it in such a way as to prove it was signed with an unaltered build signed by people we are currently already trusting?
No. For the client to sign the tx would require that the client have the private key. If the client has the private key an attacker could use that same key to impersonate the client. Don't feel bad this is very similar to DRM and countless billions of dollars have been wasted on that flawed concept that. Any security system which relies on the code running on an attacker machines to not do "bad stuff" is just feel good security. Don't feel bad this is very similar to DRM and countless billions of dollars have been wasted on that flawed concept that. The good news is Bitcoin doesn't rely on feel good security it relies on strong cryptography. You can modify your copy of the client to do anything you want (like make the next block have difficulty of 1, and a block reward of 84390482930 quadrillion BTC). You can't control how other clients will react to that and in this case they would simply ignore it as invalid data.
Still even if you could absolutely cryptographically PROVE that a tx was created by a particular client it would still be utterly useless. The term is "double spend" for a reason. There are two txs in a double spend. Proving that one of them is "legit" means nothing because the problem isn't with the legit transaction, it is with the competing one which you may be included in the next block. The "proof" that this isn't the case is tx confirmations. Nothing else has any value.
But there could in principle be something special about a tx that is "not as raw."
No, there is nothing special about "not as raw" txs either. Not today, not ever. The security model of Bitcoin doesn't rely on your client doing the right thing. In fact all other nodes implicitly distrust your node and independently verify data received from your node before relaying it. The security model of Bitcoin is strong because it doesn't (weakly) rely on an assumption that there are "good clients" and "bad clients" as indicated above that is pointless feel good security.