Hacker's log, supplemental: so in the end, the network interface lights up and all seems well with that bit. The HF counter DOES indeed work, as tested to 2.1Ghz via my HP8642B: 2 100 037 590.707 Hz (and that is using the stock XO in the AWG, and the OCXO option in my HP8642B). So that seems to work a treat.
The other interesting thing I found while snooping around the AWG's filesystem is this when I inadvertently rebooted the AWG with a FAT32 formatted SD card still in:
##### EmbedSky BIOS for SKY2416/TQ2416 #####
This Board: SDRAM is 64MB; LCD display size is: 800 X 480
Press Space key to Download Mode.!
And then after a minute this pops up, again, no input via the console or anything, just left it do it's own thing:
error found: 0010
And the AWG does not boot further. So this makes me pretty sure that there is a way to at least "externally" boot the HDG2000 series via a SD card if we have the correct FS etc on it.
AH HA! And there is MORE! If I am a good little boy and follow the nice machine's instructions, I get THIS!:
##### Boot for SKY2416/TQ2416 Main Menu #####
##### EmbedSky USB download mode #####
[1] Download u-boot or STEPLDR.nb1 or other bootloader to Nand Flash
[2] Download Eboot (eboot.nb0) to Nand Flash
[3] Download Linux Kernel (zImage.bin) to Nand Flash
[4] Download LOGO Picture (logo.bin) to Nand Flash
[5] Download UBIFS image (root.ubi) to Nand Flash
[6] Download YAFFS image (root.bin) to Nand Flash
[7] Download Program to SDRAM and Run it
[8] Boot the system
[9] Format the Nand Flash
[0] Set the boot parameters
[a] Download User Program
[n] Enter TFTP download mode menu
[r] Reboot u-boot
[t] Test Linux Image (zImage)
Enter your selection:

Cool. The two are not related of course, because you get the same "hold Space to download" prompt even without the SD card in the front.