Can you provide a little mock-up screenshot or any details about what sort of control the 'Waste Class' control is? 

It would better help to create an example. 

@MegaJerk Thank you for your response. I had despaired of ever hearing from anyone.  The field that I wanted to perform the validation on, Waste class, is a choice field drop down list, which is outside, and above my repeating section, (so it is completed before entering values in the repeating section). The Waste class has 4 options: RCRA-haz, Non-RCRA haz, Non-haz, and Universal.  If RCRA-haz or Non-RCRA haz is selected the user can enter only 1 item in the repeating section, if Non-haz or universal is selected, they can enter multiple items.

However, I ended up abandoning this design, in favor of a tabbed form with associated panels for my line items, due to the fact that the values entered in the repeating section have to be calculated by other fields in the list, and creating a calculated field which needs to perform multiple calculations on a comma separated field with multiple items was not an option.  So, while I would still like to know, if it is possible, and how, to manipulate the repeating section maximum rows property via validation, or some other method, I no longer require this solution on this current project.  It might be a good blog post for someone.