typedef struct oss_audioinfo
{
  int dev;                      /* Audio device number */
  char name[64];
  int busy;                     /* 0, OPEN_READ, OPEN_WRITE or OPEN_READWRITE */
  int pid;
  int caps;                     /* DSP_CAP_INPUT, DSP_CAP_OUTPUT */
  int iformats, oformats;
  int magic;                    /* Reserved for internal use */
  char cmd[64];                 /* Command using the device (if known) */
  int card_number;
  int port_number;
  int mixer_dev;
  int real_device;              /* This is the right /dev/dsp# device to open */
  int enabled;                  /* 1=enabled, 0=device not ready at this moment */
  int flags;                    /* For internal use only - no practical meaning */
  int min_rate, max_rate;       /* Sample rate limits */
  int min_channels, max_channels;       /* Number of channels supported */
  int binding;                  /* DSP_BIND_FRONT, etc. 0 means undefined */
  int rate_source;
  char handle[32];
  int nrates, rates[20];        /* Please read the manual before using these */
  oss_longname_t song_name;     /* Song name (if given) */
  oss_label_t label_name;       /* Device label (if given) */
  int filler[195];
} oss_audioinfo;

This ioctl is meant to go straight to the hardware, reporting only what's supported natively by the hardware. That rules out emulating SNDCTL_DSP_GETCAPS/GETFMTS, because they both depend on the intermediate channel interface. Mailed netchild & ariff for advice.