executing select * from nt:base where jcr:path like '/demo-project/%' AND (mgnl:template = 'stkEvent')
results in

javax.jcr.RepositoryException: Column 'mgnl:template' does not exist on the table 'nt:base'
	at org.modeshape.jcr.query.qom.JcrAbstractQuery.checkForProblems(JcrAbstractQuery.java:133)
	at org.modeshape.jcr.query.JcrQuery.execute(JcrQuery.java:104)

It is true that nt:base doesn't explicitly define such property, however since it doesn't restrict it's existence any of the node types based on nt:base can do so and therefore this query should be valid and JCR implementation should search through all the nodes based on the nt:base for those that define the property and have search value.