It has little to do with the cost of the game. But there is this saying, if the product is free, you are the product. So in a way, this type of behaviour for a free game would be even more suspicious.
As I said, I am old fashioned. But it sounds to me like an overly complicated method of promoting your new game with your old game. If I understand correctly, you plan on releasing a previous web game as a download game and giving people playing the old game a bonus for the gameplay, if they click on your promotion link. Wich would leave a sour taste in my mouth as a player.
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Since you specifically make a promotional release of the old game, why can't the advertisment be static? And why has it to give a boost. Wait a minute. If reread, the players do not even have to visit the link, they only have to view a screenshot?! I do not really understand why this has to be dynamically implemented.
Would it not be easier to just tell it outright, like, this is xyz - promotional release, with big letters in menu pointing to the new game with links and planned release and informing the players, that they can sweeten the waiting time by playing your time tested old game? What will happen with your promotional hard coded phoning home in a year, when someone starts the old game?
You could consider giving a permantent boost for playing the promo version, what with having it clustered with references of your new game. After all, this download version will get downloaded and assuming your new game still exists in three years, you might still benefit from promoting it with the old game. But you will probably not maintain your little self jury rigged phoning home and downloading files (I did mention, that I do not trust indie devs to do such things, did I ;-)
Oh, and all this obviously depends on people knowing and being interested in your old web game in the first place. Those people would probably be interested in your new game anyways. So maybe a simpler solution might do.