ZmnSCPxj suggests that if it isn't possible to make a graftroot signature independent of the outpoint, then the functionality is greatly reduced. The main gains of graft are being able to delegate before coins exist and making the conditional transfer atomic. The proposal suggests defining a transaction nVersion reserved for Graftroot, which disallows transactions with that nVersion in blocks. If a next-SegWit-version P2WPKH or P2WPK is spent, and the top witness stack item is a signature with SIGHASH_GRAFTROOT flag, then this is a Graftroot spend. The imaginary tx has the Graftroot-reserved nVersion. This lets users select whether committing to a specific outpoint is needed or not, independently of Graftroot.