Hi,
I'm not familiar to section 508 accessibility compliance checking, but as far as I know, it is hard to customize Check-In policy in Visual Studio Source Control and SourceSafe.
And I would like to suggest you try to Visual Studio Team Foundation Server which allows you to create a custom check-in. You can refer to this walkthrough:
Walkthrough: Customizing Check-in Policies and Notes
Hope this helps,
Nancy
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