The WinEnumObjectClasses function will return a list of all workplace object classes that have been registered.

#define INCL_WINWORKPLACE
#include <os2.h>

POBJCLASS    pObjClass;  /*  Pointer to object class. */
PULONG       pSize;      /*  Length of the pObjClass buffer in bytes. */
BOOL         rc;         /*  Success indicator. */

rc = WinEnumObjectClasses(pObjClass, pSize);