But with TXHASH and CSFS, both options are possible, while with a sighashtype only the at-sign-time option… To emulate a sighashtype, the user would pass a TxFieldSelector (or whatever you call the input to TXHASH) and a signature and then the script would generate the hash value using the user’s input and then validate the signature.
One could actually argue that this is a more pure implementation of OP_CHECKSIG in Bitcoin Script…