Context: assuming from above comments that a BSDish
libc is meant.
I think it's been looked into, but
libc tends to be tightly tied to a given kernel (
glibc has an abstraction layer, which allows it some portability but causes the usual problems that an abstraction layer causes) and making BSD
libc work with a Linux kernel would require a near complete rewrite. key system services are very different between the two systems (one example: BSD
libc assumes that there are no pipes/FIFOs, because BSD uses socketpairs instead; conversely, Linux doesn't support pipe-compatible socketpairs).
Going the other direction (Debian has an experimental Linux userspace on a FreeBSD kernel, I think) is possible due to
glibc's portability layer.