Do you know that those who are addictive to gambling are the ones who actually want to win real money. The addiction is not about playing gambling only but the real addiction is to earn money through gambling. So if you keep this in mind, then gamblers would never agree to play with fake money. If this was the case then gamblers had a lot of option to play at other sites which are non-gambling one. No one will agree to play with fake money as per my thinking.
In general, I agree that a very small number of gamblers will be interested in playing for fake money. But I want to clarify that gambling addiction is not just about the desire to make money, it is precisely the irresistible desire to continue the game, no matter what happens. I think that if you choose people with a gambling addiction, ask them about the amount that would suit them as a "final" win (having received which they would stop playing) and give them this money, then most of them will continue to play anyway.