You are right that orfeo toolbox is not yet available in the 64 bit Version of OSGeo4w installer. You can compare the package listings here:

http://download.osgeo.org/osgeo4w/x86/versions.html

http://download.osgeo.org/osgeo4w/x86_64/versions.html

The 64-bit version is still experimental, with packages missing or not working as expected.

However, it is possible to use the 32-bit installer to install orfeo toolbox in C:\OSGeo4W (while the 64bit QGIS is in C:\OSGeo4w64), and nethertheless enter the paths to otb in 64-bit QGIS 2:

    C:\OSGeo4W\apps\orfeotoolbox\applications
    C:\OSGeo4W\bin

This even works with the standalone installers.