What benefit do you hope to gain from an arbitrarily long nonce?
The design I sketched out extended the 96-bit GCM nonce to 224 bits, which is longer than the 192 bits of XSalsa and XChaCha. That's also the maximum that's supported by the algorithms as used.
If we supported arbitrarily longer inputs to AES-CBC-MAC, it's going to get mixed down into an AES block (128 bits long) anyway, so the benefit of arbitrary-length extensions over a 128-bit extension is unclear to me.
The design I sketched out extended the 96-bit GCM nonce to 224 bits, which is longer than the 192 bits of XSalsa and XChaCha. That's also the maximum that's supported by the algorithms as used.
If we supported arbitrarily longer inputs to AES-CBC-MAC, it's going to get mixed down into an AES block (128 bits long) anyway, so the benefit of arbitrary-length extensions over a 128-bit extension is unclear to me.