I'll answer the easiest of your questions first — the existing Special By-Law for the solar hotwater system can be amended so that it applies generically to the entire common property (roof); use this Form https://www.lpma.nsw.gov.au/__data/assets/pdf_file/0017/25550/15CB_v3-1.pdf
With regard to the wording of the amended Special By-Law, you can be as prescriptive as you like concerning the elevation of the systems, obstructing views, the need for structural engineers' reports, damage during installation, responsibility for on-going maintenance / repairs etc etc; just don't leave room for ambiguity and interpretations.
Your O/C is actually granting to its Proprietors a license to use the Common Property, whether that's the roof or the ground, so the requirements for the heat-pump could easily be incorporated; just keep the wording simple.