found that, when wiced_bt_gatt_send_notification() is called and congestion is about to occur, the function returns with a congestion notice, but also there is a GATT callback that indicates congestion. this can be used to accomplish flow control.
the wiced_bt_gatt_evt_t member of the callback is a 5, which isn't defined, but when this occurs, we check p_event_data->congestion.congested to find out if congestion is occurring.
when congestion lets up, the same event is called and p_event_data->congestion.congested is false.
Update: unfortunately I find that if I attempt to use "congestion" as a flow control, the BLE ends up congested most of the time and unable to perform functions like listing out characteristics which gives major problems when clients try to connect.
My temporary solution is to use indications, which require an ACK from the central. This requires more radio comm & power consumption than I'd like so still hoping some ability might be there to check buffer pool/queue status to make sure we're not congesting.