I often read this forum - its full of amazing ideas! It is interesting that this topic came up - I run a company making messenger bags for cyclists, and we have just released a set of bags that have maps printed on the liner, upside down, so when you get lost, you can swing your bag around and just find out where you are! There are printers around now that can print on basically anything (wood, metal etc..) so there is no reason not to print on whatever you want. If ink wont hold, you could always screenprint using a medium that will adhere to the fabric in question.
We started out by looking at soliders in the world wars, who had silk maps sewn into their clothing in case they found themselves lost in enemy territories, but t-shirts is a great idea - you would form a set of t-shirts from each trail you walk!
Anyway, feel free to have a look at our bags here: