You can do it without a MMC by downloading first to your N900's data card. Using your 770 in USB host mode you can connect the 770 and the N900 to make the transfer. This will have to be done with OS2008HE, which has X-Terminal built in. You need XTerm to mount the N900 card on the 770, which does not automount USB storage.

[EDIT: If you no longer have your N900, you could download to a USB memory device via another computer, and then attach the USB memory device to the host-mode 770.]

(You say something about R&D mode, so it sounds as though you are using the Linux flasher. The flasher can be used to set the 770 into USB host mode. You will have to supply USB power through a Y-cable, available for a few dollars on eBay. The 770 has no internal power to the USB transceiver.)

Alternatively, If you get yourself a cheap MMC you can download first to another computer and then transfer the file by USB to the 770's MMC .