If we try to start a testnet with the flag --graffiti on the validator client by adding the following under participants in network_params.yaml (with Lighthouse and Geth combo):

vc_extra_params:
            - --graffiti=testing123

Kurtosis wouldn't start:

error: the argument '--graffiti <GRAFFITI>' cannot be used multiple times
  
  -Usage: lighthouse validator_client [OPTIONS]

So it seems to be unable to supply user configured graffiti on the validator client. This could be good to have, e.g., when testing a feature to append client version info in the graffiti: https://github.com/ethereum/execution-apis/blob/main/src/engine/identification.md