The problem is that luminance textures aren’t color-renderable, so the FBO is not complete.
The following base internal formats from table 3.15 are
“color-renderable”: RGB, RGBA, FLOAT_R_NV, FLOAT_RG_NV,
FLOAT_RGB_NV, and FLOAT_RGBA_NV. The sized internal formats
from table 3.16 that have a color-renderable base internal
format are also color-renderable. No other formats, including
compressed internal formats, are color-renderable.
I would not bet on the specification. On ATI, the FBO is complete when one of these formats is attached to it: http://www.fi.muni.cz/~xolsak/fbo_atix1700.txt
I’ve tried a few of them (LUMINANCE16F, LUMINANCE_ALPHA16F, LUMINANCE32F, LUMINANCE_ALPHA32F) and it worked. It takes the red component for luminance too.