You could do something such as:
<rules>
<rule name="LowerCaseRule1" stopProcessing="true">
<match url="[A-Z]" ignoreCase="false" />
<conditions>
<add input="{URL}" negate="true" pattern="^~/myfolder$" />
</conditions>
<action type="Redirect" url="{ToLower:{URL}}" />
</rule>
</rules>
or... you could create another rule that does essentially the opposite for the specific match:
<rules>
<rule name="LowerCaseRule2" stopProcessing="false">
<match url="^~/myfolder$" ignoreCase="true" />
<action type="None" />
</rule>
</rules>