Possibly it could contain the address at which you could access the workbook online, and maybe print it out, although even such a trivial inconvenience could still be a barrier to a lot of people.
Bothering to access the workbook online could be a practice test of instrumental rationality in and of itself.
And if you explicitly frame it as such, the entire book has primed them to pass this particular test.
Note: This was originally posted in the discussion area, but motions to move it to the top level were made.
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My own desire to improve my rationality coupled with some posts criticizing LessWrong not too long ago led to an idea. For reference, the posts I mean are these: