Why not format it as a checkbox set so that clicking it enters a 1 and clearing it clears the field?
All you need is a value list of one custom value: 1. You then resize the field to hide the 1 from view so that only the check box is visible. In FileMaker 1 = True and 0 or empty = False so this produces the desired boolean values.
It is also possible to set up a text object with a conditional formatting that is also a button so that clicking it modifies the value in the field and conditional formatting makes the text object appear or disappear, but that's a lot more work to set up so I wouldn't do that unless I had a significant reason for not using a single value checkbox set for this.
Yesterday, at DevCon 2013 David Knight from Angel City Data provided an example of just what the original post here is talking about. In his example, he uses the Inspector's Data tab to designate a check mark for the 'True' value by Option + v key command on the Mac, alt +251 in Windows or Unicode 10003. I don't know how to apply the Unicode, so I'm using Option + v on my Mac. He also incorporates a simple script, but I'm reserving comment on that part until I can make it work right for myself.