Ah, I found a work around that works and only is a tiny bit cumbersome - add a filter with a very low 'precedence'
to the default case. (You can find out the schema precedence by dumping the schema with "sysmon[64].exe -s".)
Here is the final working test case:
<Sysmon schemaversion="4.50">
<EventFiltering>
<RuleGroup name="ProcessCreate - Include" groupRelation="or">
<ProcessCreate onmatch="include">
<OriginalFileName name="Original filename catch" condition="is">net.exe</OriginalFileName>
<Rule name="Default catch" groupRelation="and">
<Image condition="is">C:\Windows\System32\net.exe</Image>
<ParentCommandLine condition="is not">bogus_value</ParentCommandLine>
</Rule>
</ProcessCreate>
</RuleGroup>
<RuleGroup name="ProcessTerminate - Include" groupRelation="or">
<ProcessTerminate onmatch="include">
<!-- Empty rule set -->
</ProcessTerminate>
</RuleGroup>
</EventFiltering>
</Sysmon>