Thanks.  I didn't know this interface existed.  This might be just what I'm looking for.

Base on your suggestion and the example code in the docs, I wrote this test program:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/usbdi.h>

int main(int argc, char *argv[]) {

	int rc;
	usbd_connect_parm_t parms;
	struct usbd_connection *connection;

	usbd_device_ident_t ident;
	usbd_funcs_t funcs;

	usbd_device_instance_t instance;
	struct usbd_device *device;

	usbd_device_descriptor_t *descriptor;
	struct usbd_desc_node *node;

	memset(&parms, 0, sizeof(parms));
	memset(&funcs, 0, sizeof(funcs));

	funcs.nentries = _USBDI_NFUNCS;
	ident.vendor = USBD_CONNECT_WILDCARD;
	ident.device = USBD_CONNECT_WILDCARD;
	ident.dclass = USBD_CONNECT_WILDCARD;
	ident.subclass = USBD_CONNECT_WILDCARD;
	ident.protocol = USBD_CONNECT_WILDCARD;
	parms.vusb = USB_VERSION;
	parms.vusbd = USBD_VERSION;
	parms.ident = &ident;
	parms.funcs = &funcs;
	parms.connect_wait = 2;

	if ((rc = usbd_connect(&parms, &connection)) == EOK) {

		//attach to USB0
		int busno;
		int devno;
		for (busno = 0; busno < 10; ++busno) {
		    for (devno = 0; devno < 64; ++devno) {
				memset(&instance, USBD_CONNECT_WILDCARD, sizeof(instance));
				instance.path = busno;
				instance.devno = devno;
				if ((rc = usbd_attach(connection, &instance, 0, &device)) == EOK) {
					if ((descriptor = usbd_device_descriptor(device, &node)) != NULL) {
						printf("Found device\n");
					}
				} else {
					printf("Bus %d device %d: %s\n", busno, devno, strerror(rc));
				}
		    }
		}
		rc = usbd_disconnect(connection);
	}
	return EXIT_SUCCESS;
}

All calls to usbd_attach return ENODEV, except the call for busno = 0 and devno = 1.  That call returns EBUSY.  I have a
 USB drive plugged in and the values corresponds to the report given by "usb". What am I doing wrong?

> You could do a shared access scan for the device by connecting 
> (usbd_connect())to the USB server but not providing any callbacks and 
> setting the device ident to use wildcards. You then use usbd_attach() to 
> loop through all the devices (see usbd_attach() docs for notes on 
> "Looping") and check the instance of the attached device for a dclass of 
> 0x06, a subclass of 0x08 and maybe even a protocol of 0x50 (that's what 
> usb -vv showed for my umass device, your mileage will vary though). You 
> could then increment a counter to track how many umass devices are 
> plugged in at the time or just bail out at that point and return status 
> to your script etc.
>