Is there any way to get pkgsrc to _not_ rebuild all the dependencies
  for a dynamic library (same version being rebuilt because of zlib
  issues)? From reading the guide, it seems proper usage of ABI/API
  DEPENDS would help solve this, but there doesn't seem to be any
  distinction between static dependencies (any change requires a rebuild)
  versus dynamic dependencies (only rebuild if API/ABI changes) in terms
  of how executables are linked.

Not what you asked, but check out pkgtools/pkg_rolling-replace - this
does make replace in topological sort order.  As Joerg hinted, many of
us have talked about better dependency tracking for along time, and this
could be implemented by not setting unsafe_depends in make replace when
there is no ABI change.  But it's not there yet.  You can do make
replace and manually remove the unsafe_depends.

Or, you may find, as do I, that letting pkg_rolling-replace do the
rebuilds is not that annoying, because your system works in the interim.