In addition to what Progressive has posted here is the link to acceptance of foreign STC's.
Civil Aviation Safety Authority - AWB 00-16 - Acceptance of Foreign Supplemental Type Certificates (STCs)
"AWB 00-16 Issue 1, 17 January 2005
Effectivity
To all owners, operators and maintainers intending to incorporate an STC issued by a foreign country.
Background
CASR 21.114 Foreign STCs states: A certificate (however described) for an aircraft, aircraft engine or propeller that is issued by or for the NAA of a recognised country and is equivalent to a STC that could have been issued by CASA (a foreign STC) is taken to have been issued by CASA for these regulations. CASR 21.114 provides the Australian aviation industry with ready access to certain modifications that have been approved by recognised overseas authorities. This Bulletin clarifies some issues relating to this regulation, to assist the consistent use of foreign STCs."
A number of people getting together to have an STC drawn up where one does not exist rather than individual EO's could make sense. Or asking the stc holder to amend by adding additional types.