I'm afraid that Windows Installer does not support this feature.
However, this could be done by using a custom action scheduled in the UI sequence. This custom action will run on the "LicenseAgreementDlg" dialog and it will search for the handle of the scrollable edit. Once it is found, you can check if and when the EULA is scrolled down.
I need to do exactly this, force the user to scroll through the entire EULA down before allowing them to accept the license (or enable the accept radio button). Could you please provide a more specific instructions ? I am new to your product.
I'm afraid that I cannot give you specific instructions for creating this custom action. This is a very complex operation and it exceeds my knowledges.