If you buy the Steam (and probably) GoG version today, you will get access to the Mac (and Linux) versions for free as well. The Linux version, the Enhanced Edition, is expected to come out at the same time - October 27th.
There are no plans to give PC users the Last Trick bag, that's a console exclusive because console users won't pay attention unless there are special shiny beads they can pre-order. Perhaps at a later date that stuff could come to the PC.
