GOG Galaxy?

More than likely the client could be easily made portable (technically, I'm not thinking about whether the license would allow it at this point in time).

Games installed through the client are an entirely different matter. Due to the sheer number of them it would be a massive (nearing impossible) task for a small team like ours, and is likely not viable anyway as someone with a few larger games (or a larger number of smaller games) installed would likely have to wait an unreasonably large amount of time for Galaxy to start and close.

Then if the user ran the games without going through the client, most games would not be able to access existing game data, so save games and configs would reset to default while bypassing the client.