Libraries loaded with RTLD_LOCAL will still have their symbols resolved against those from already loaded libraries as well as the main executable. Looks like there is dlmopen() in glibc since ~2005 though and there is libcapsule [0] which makes it actually usabel - neat.

[0] https://github.com/dezgeg/libcapsule

Plus also linking with groups, from memory, -Wl,-Bgroup although this may be a Solaris ism.