Can I install engineered wood flooring direct on the joists (my preference) or must it be installed on top of an existing floor?

On a subfloor (or over both sub and finished floor), and perpendicular to the joists.

Here is a typical spec sheet for engineered flooring.
http://www.homedepot.com/catalog/pdfImages/39/392ea2e7-be75-465b-a1f8-6c71ec512d5a.pdf

Note the statement that this type product is not considered a structural component. Also, the code for your vicinity most likely requires a subfloor.
I would say- in reply to your question-absolutely not.

Thanks for the prompt feedback and the link to the document. Much appreciated.