I found this in the source in the brcm80211 directory, in wl_cfg80211.c.
switch (type) {
case NL80211_IFTYPE_MONITOR:
case NL80211_IFTYPE_WDS:
brcmf_err("type (%d) : currently we do not support this type\n",
type);
return -EOPNOTSUPP;
So it looks like monitor mode is NOT supported.
Rgds,
Dave.