The uclibc library is *not* as other user-space packages you can just cross-compile and push to your rootfs to make it work. It’s the base of the whole toolchain (along with the GCC compiler and binutils packages). This means that the cross-compilation of this library is done as part of the toolchain itself creation, not at a later stage where you use the already generated toolchain to build other packages and libraries.
In case for DEL Digi used *crosstool-ng* project to build the basic toolchain. This project does its magic with GCC+UCLIBC internally to build the toolchain. It’s not an easy process (in fact that’s why the project exists). If you update the uclibc component then all the user-space packages need to be built again.
Not sure which is the overall background of the problem, but if what’s failing is just an specific in-house developed application, the customer may use *crosstool-ng* the generate a toolchain with a newer version of uclibc and then use that toolchain to *statically* build its application. This will allow to run the static app with a DEL rootfs that has an older version of uclibc (the one provided with DEL).