I am curious whether scenarios can include elements (and, in particular, monsters) that are outside the SRD but are Open Content? I noticed that one of the earlier open calls specifically included the nabasu, but they didn't seem happy with the results.
Also, if it is "allowed," is it still discouraged barring a very good reason?
Partially, it is discouraged because non SRD monsters need their whole stat-block reprinted: It is fair to assume someone has a Monster Manual or d20srd.org available, but not to assume they have the Tome of Horrors or Advanced Bestiary. These stat blocks eat pages and word count when you could instead cite a core monster by page number.
If you have a really good idea that uses a 3rd party open source monster or rule please include it with a reference to where you found it. We're open to open source content in the scenarios.
Thanks a bunch for the quick answers!