I'm sure if it was possible they would have added it already to HB and as it is not added, I don't think it's possible or it's a big risk word travels quick around the Internets
It's not a problem when it it for a limited time. See it like a marketing promotion, like 1 day free after you will surely buy it if you like it. Many people just don't buy because they don't know what to expect. That was my case before i took a risk of buying something totally unknown. If a had tryed it for free long ago i would be a paying customer much earlier.