We haven't worked on a unit like for several years, so that should tell you something. Some of those games may get worked into the main Genesis products at some point in the future, though.
I actually loved the Master System portable... I believe it was called something like the "Arcade Gamer Portable"... which, to be honest, is an absolutely horrible name for something filled with Sega Master System/Game Gear games. The originals weren't even the right color scheme (black and yellow??? seriously???). I can understand why something like that didn't sell well. I think it would be much better for the SMS/GG games to be integrated into the Sega Genesis style products, as the Master System didn't sell well here in the US. Consequently, there isn't a lot of mass market nostalgia for a Master System inspired device.
AtGames had them on their website till fairly recently (I purchased one of them as a spare for $20 + shipping a few months ago directly from the AtGames website). Now that AtGames has updated their website into the "Flashback Zone", they don't appear to be available any longer.