Is there a convention for lines separating areas in circuit diagrams?
NEMA has some guidelines for electrical schematics: 



There are other options besides dashed lines- especially in the modern era. We can use colors, filled boxes of various shapes and colors with various types of borders etc. Absent a relevant set of standards, I think it's best to just make it obvious and simple, not too ugly, and annotate it as Ignacio comments. 

Try to to make it difficult to confuse with the boxes that are used in hierarchical schematics or with component symbols that may show internal circuitry.