Again there is absolutely no documentation out there on this, WiX provides out of the box support for doing this though. Just define a custom action into wixca.dll with entry point PrintEula
<CustomAction Id="PrintEula" BinaryKey="wixca" DllEntry="PrintEula" Return="ignore" Execute="immediate" />
And on the LicenseAgreement dialog, define the EULA in a ScrollableText control with ID “LicenseText", this is how WiX knows what text to print.
<Control Id="LicenseText" Type="ScrollableText" X="201" Y="62" Width="254" Height="66" Sunken="yes" TabSkip="no">
<Text SourceFile="..\License.rtf" />
</Control>
Simple!