You can accomplish this by running a cable in your environment from where your keyboard's MIDI comes in to Logic directly to the instrument you need to play. This will not allow you to record the piano, but you can at least play it at all times, regardless of what track is selected in the arrange page.
If you want to record your piano parts as well as be able to play them at all times, well, that'll take a different approach.
A few questions for you...
1) what keyboard controller are you using?
2) do you know how to change the controller's Transmit Channel between, say, 1 and 2?
3) do you have any programmable buttons or switches on your controller that you're not currently using for something else?
4) do you have any experience tinkering around in the Environment?
Upon receiving answers to those questions I can offer you some suggestions.