For Poc-1, calling the author_submitExtrinsic RPC with "params": ["0xfff745..."] works as expected.

Currently on master, the serialisation expects an array of bytes, i.e. the above needs to be encoded as [255, 245, 69, ...] This doesn't align with any of the other RPC requests or results where "blobs of data" are always expected/returned as hex-encoded strings.

If a "rich" encoding is desirable as an RPC, then it should be done with a second RPC.