No ideas on 'easy' way to identify, but here are a few more to add to your list:
-- by name
+ sybsystemdb
+ dbccalt
+ sybpcidb
+ sybcatdb (Workload Analyzer)
+ sybdsamdb (Heat Maps)
-- by status bit
+ user defined tempdb's
-- oddballs (depends on your definition of 'system' db)
+ Repserver RSSD's
+ user defined scratch dbs (archive db support)
+ IMDB template db's