I found some (unused..) USB-related U-Boot parameters in some of the files contained in the ‘firmware’ zip file, and tried these, to get the APC to boot from USB instead of MMC/micro-SD..   The USB connection has to be to the top connector in the smaller of the two ‘towers’ on the board (the one without the ethernet port)..   This presents /dev/sdb1 (FAT) and /dev/sdb2 (EXT2) (and ‘usb 1.1’ for the fatboot parameters)

scsi 0:0:0:2: Direct-Access     Kingston FCR-HS219/1      9745 PQ: 0 ANSI: 0
sd 0:0:0:1: [sdb] 3842048 512-byte logical blocks: (1.96 GB/1.83 GiB)
sd 0:0:0:1: [sdb] Write Protect is off
sd 0:0:0:1: [sdb] Assuming drive cache: write through
scsi 0:0:0:3: Direct-Access     Kingston FCR-HS219/1      9745 PQ: 0 ANSI: 0
.................
sd 0:0:0:1: [sdb] Assuming drive cache: write through
 sdb: sdb1 sdb2
sd 0:0:0:1: [sdb] Assuming drive cache: write through
sd 0:0:0:1: [sdb] Attached SCSI removable disk

Basically, the results were consistent with the previous scenario.    The APC does boot (eventually..!.) from USB:

..............
 usb_request_sense
 usb_request_sense
 usb_request_sense
 usb_request_sense
 usb_request_sense
 usb_request_sense
 usb_request_sense
 usb_request_sense
 Device NOT ready
   Request Sense returned 02 3A 00
4 Storage Device(s) found
part_offset : 800, cur_part : 1
reading uzImage.bin   ..............etc....

But – as before the device still forces the base filesystem to be at /dev/ram0. with the resulting basic Linux+Busybox only..