Para quem chega aqui pelo Google, consegui que funcionasse com a seguinte regra:
<rewrite>
<rules>
<clear />
<rule name="Force HTTP" stopProcessing="true">
<match url="(.*)" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false">
<add input="{HTTPS}" pattern="on" />
</conditions>
<action type="Redirect" url="/{R:1}" redirectType="Temporary" />
</rule>
<rule name="Allow Local Resources" stopProcessing="true">
<match url=".*" />
<conditions logicalGrouping="MatchAny" trackAllCaptures="false">
<add input="{REQUEST_FILENAME}" pattern="SiteMaintenance.png" />
<add input="{REQUEST_FILENAME}" pattern="Stylesheet.css" />
</conditions>
<action type="None" />
</rule>
<rule name="Check Is Root" stopProcessing="true">
<match url="^.+$" negate="false" />
<conditions logicalGrouping="MatchAny" trackAllCaptures="false" />
<action type="Redirect" url="/" appendQueryString="false" redirectType="Temporary" />
</rule>
</rules>
</rewrite>
Não faço ideia se este é o melhor caminho, mas funcionou para mim.
:)