It appears that in V2R1U3, certain MetaClasses are "categorized" as "usual" MetaClasses, which are the only ones to appear in the drop-down list of the simple query tool on the web client. To see all the MetaClasses in the drop-down, you have to open the advanced query assistant, and check the "View all objects types" option.
Is there a way to "categorize" other MetaClasses as "usual" so they would show in the web simple query drop-down? Is it possible to tune this list per Profile, i.e.: certain MetaClasses are "usual" for Profile A, while others are "usual" for Profile B?