I have found that even if I'm in a situation where I cannot access pen or paper or anything else for that matter, I can still nearly achieve the same effect through visual imaging of creating tangles. I have created so many Zentangles that I can visualize in my mind's eye the feel of the paper, the scratching of the pen across the surface, seeing that moment when the ink flows from the pen, and of course the patterns for each tangle. My mind automatically allows me to go to that safe haven, if you will, and creates a very similar response. The key is to see every detail, smell, feeling... I think one of these days I will try visiting one of my Zentangles in an imagined 3-D world - really get to know the tangles from a different perspective :)
As a challenge and to help deepen your experience of the Zen I encourage you to sit for awhile with a Zentangle tile; run your fingers over it and get the feel of it; notice every indentation of the paper in it's mini mountain and valley depths; look at it from every angle and direction - spin it in your hands. Now close your eyes and try to see it in detail as you hold it in your hands. Can you see it clearly? Every nook and cranny? Wording? Shadows and highlights?
Now pick up your pencil and create a string - really be present. Visualize yourself as the graphite point. Taking a journey that you don't know where it will lead. You are climbing the peaks and valleys of the paper and leaving a light trail of your footsteps behind you.
Now pick up your pen. Notice the detail of the pen. How it feels, how it looks. As you take the cap off the pen imagine that your are lifting the caged doors to your creativity - your are about to be freed :)
As the pen tip hits the paper, again, imagine the new little world you are delving into, this time you are creating intricate trails in the mountains and valleys. Watch as the ink flows out of the pen - it's leaving a trail of the past and ahead is your future yet to be explored, you are in the moment.
As you practice this, try closing your eyes afterwards and recreate this in your mind's eye as detailed as possible. With practice you will be able to achieve the same state of relaxation by taking a deep breath, visualizing the details and the person or thing giving you difficulty will never even know how you are remaining as cool as a cucumber :)