transmission loss with microsd extender
I can't say with any certainty, but I think that for 20 to 30 cm it should work, as that length is just a small fraction of the wavelength of the data frequencies (around 10Mhz). But consider to use a ribbon cable with twice as many signals as you need (plus a couple of more) and connect every other wire to ground so that they decouple the electric field between the adjacent wires (each signal wire sees a ground wire at each side). 

Use a couple of extra wires for the power and I suggest to put a small ceramic (or plastic) capacitor (something like 100n to 1u) between the VCC and GND at the microSD card's end to supply the drivers with some energy to make clean transients. 

The cable will have some electromagnetic emissions, I'm afraid. But probably not so much that it would become a problem.