You can use the linker to preload `nss_wrapper`[0] and overwrite anything using `nss`.

Here's an example:

  > cat hosts
  198.51.100.33 test.tld
  2001:db8::33 test.tld
  198.51.100.12 test4.tld
  2001:db8::12 test6.tld

  > LD_PRELOAD=/nix/store/sw2r0gpi9c9rsvqgvi4906yxh948ydsv-nss_wrapper-1.1.16/lib/libnss_wrapper.so NSS_WRAPPER_HOSTS=hosts getent ahosts test.tld
  198.51.100.33   DGRAM  test.tld
  198.51.100.33   STREAM test.tld
  2001:db8::33    DGRAM
  2001:db8::33    STREAM

  > LD_PRELOAD=/nix/store/sw2r0gpi9c9rsvqgvi4906yxh948ydsv-nss_wrapper-1.1.16/lib/libnss_wrapper.so NSS_WRAPPER_HOSTS=hosts getent ahosts test4.tld
  198.51.100.12   DGRAM  test4.tld
  198.51.100.12   STREAM test4.tld

  > LD_PRELOAD=/nix/store/sw2r0gpi9c9rsvqgvi4906yxh948ydsv-nss_wrapper-1.1.16/lib/libnss_wrapper.so NSS_WRAPPER_HOSTS=hosts getent ahosts test6.tld
  2001:db8::12    DGRAM  test6.tld
  2001:db8::12    STREAM test6.tld

  > LD_PRELOAD=/nix/store/sw2r0gpi9c9rsvqgvi4906yxh948ydsv-nss_wrapper-1.1.16/lib/libnss_wrapper.so NSS_WRAPPER_HOSTS=hosts curl -v test.tld
  * Host test.tld:80 was resolved.
  * IPv6: 2001:db8::33
  * IPv4: 198.51.100.33
  *   Trying [2001:db8::33]:80...
  *   Trying 198.51.100.33:80...

[0] https://cwrap.org/nss_wrapper.html