First, there was never such plan. Nobody ever made such project. Especialy not Austrian Empire, they were wanting to expand there themselves. Russia was supporting Serbia, but it was supporting Romania, Bulgaria an Greece as well. Why they just decided to make "Greater Serbia", why not Greater Bulgaria, Romania…
Consept of "Greater Serbia" was mentioned first time 1903 in Austro Hungarian propaganda.

What about Garašanin, Zach, Načertanije, 1840s.? AFAIK Načertanije was official programme of two serbian governments and one montenegrin (Petar II. P. Njegoš). At least, croatian historians consider that plan, among others, as plan for Greater Serbia. I think that S. Marković (serb. politician and socialist) was very critic about that plan.