There are ways to switch glibc other than "rewrite every binary" and "full-on containers".
In particular, if you need to replace not just glibc, but also a bunch of system libraries (pretty common case for complex apps), it's often easier to unshare(CLONE_NEWNS), followed by bind-mounting over new /lib64 and /usr/lib to override specific directories. This is much lighter than full-on containers, and allows overriding any specific directories - for example if your app looks at /usr/share/appname, you can override it too.
This method has a bunch of upsides: you can use binaries unmodified, subprocesses work, and hard-coded data locations can be taken care of as well.
In particular, if you need to replace not just glibc, but also a bunch of system libraries (pretty common case for complex apps), it's often easier to unshare(CLONE_NEWNS), followed by bind-mounting over new /lib64 and /usr/lib to override specific directories. This is much lighter than full-on containers, and allows overriding any specific directories - for example if your app looks at /usr/share/appname, you can override it too.
This method has a bunch of upsides: you can use binaries unmodified, subprocesses work, and hard-coded data locations can be taken care of as well.