What about this one? I basically copied it from second thread and modified few characters:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt"
exclude-result-prefixes="msxsl"
xmlns:wix="http://schemas.microsoft.com/wix/2006/wi"
xmlns:my="my:my">
<xsl:output method="xml" indent="yes" />
<xsl:strip-space elements="*"/>
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match='wix:Component'>
<xsl:copy>
<xsl:apply-templates select="@*"/>
<xsl:attribute name="MultiInstance">
<xsl:text>yes</xsl:text>
</xsl:attribute>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
note that you might need to tune it a little bit more, depending what heat arguments you're currently using to generate wix files.
Alternatively you can download WiX heat source and add the argument yourself. In theory should be pretty easy.
WiX Installer: using xslt with heat.exe to update attributes