Is there a tuning parameter for MySQL that allows you to set an on-disk gap between non-sequential primary keys?
Your understanding is completely wrong, so the question does not really make sense. Rows beyond the value of the point of insertion are not rewritten with insertion. Rows aren't even re-ordered within the page level for insertion; insertion happens into empty space within the page, and the rows are linked together in ascending order through a singly-linked list.

It may be informative to check out this series of blog posts I wrote: http://blog.jcole.us/innodb/