No. SD cards do not support MMC mode. Both MMC and SD has common ancestor which is SPI. And Wikipedia says "All memory cards must support all three modes, except for microSD where SPI is optional." http://en.wikipedia.org/wiki/Secure_Digital_card

But anyway this does not matter since SD mode has 1 bit mode too so it can in theory run in MMC slot in N770 (6 wires) in SD mode so SPI mode support is not needed.

N770 kernel has even optional support for 4 bit mode (if bootloader tells it that the device can do it) so maybe there are newer N770 devices in the wild which do support 4 bit mode i.e. have all pins in the connector which are needed for faster 4bit SD (and MMC) mode. My N770 (november 2005) has only 6 metal pads in the slot so it definitely can't do 4 bit mode.

Can anyone with really new N770 device say if you have more metal pads in the slot (8 or 9)? It may be that such device did not ship at all, there are other things in kernel targeted for devices that never shipped so this wouldn't be surprising.