While that could work for packages in -extras, it won't work for cssu stuff. And core system components (cssu stuff) are the major goal for thumb, as they usually stay resident in memory. And for packages which have assembly code an additional manual work is needed to make them thumb compatible. Moreover some of the packages recognise "thumb" in DEB_BUILD_OPTIONS (modest for example ) and add the appropriate compiler flags, so no secial debhelper is needed, while the others does not have a way to affect compiler flags from debian/rules.

@don_falcone: "-+r" seems to not work either, as it affects only writes