I have skimmed 802.11-1999 through clause 7 and have not found anything that specifies one MAC address (BSSID) for each SSID. It seems illogical to me that each SSID should share a single BSSID per AP.
I haven't seen anything in the standard that disallows multiple SSIDs on one radio module. Wouldn't this be analogous to putting multiple virtual routers or multiple VLANs on a single link?
What is your work around to the single MAC address problem and is the manufacturer working on a solution?