A number of plants in the Asclepiadaceae family are used medicinally and have been flagged as having pharmacological potential, but almost no research has been done so far. Of the Hoya genus, Hoya diversifolia, coriacea, and coronaria are of interest.
coronaria - the latex is used to induce vomiting (Indonesia)

diversifolia - the leaves are mixed with hot water to ease rheumatism pain (Cambodia, Laos, Vietnam, Malaysia)

Wiart, Christophe. Medicinal Plants of Asia and the Pacific. Boca Raton: CRC/Taylor & Francis, 2006. Print.

This has been brought up in a past thread and Mike had done some research into it - but I can't find the thread now.