I've seen quite a few cases in Canada where a bush contract inculded a contra deal for placement of the customer's name/logo on the helicopter for the duration of the contract. However, I don't think that the customer paid extra for this - the operator was more than happy to do it in return for getting the contract.
Conversely, I can't see why any charter customer would want to be flying around with someone else's "flag" on the helicopter. Especially in corporate charter, this idea might lose you more charter revenue than it will bring in advertising fees.