jeez. it works

package.nix

@@ -5,17 +5,29 @@
 {
   name,
   pkgList,
-  fileGlobs ? [ "bin/*" "lib/*"],
+  fileGlobs ? [ "bin/*" "lib/*.so.*"],
   fileFindPredicates ? "-type f",
   pkgProcessor ? pkg: pkg,
 }:
   let
   processedList = map pkgProcessor pkgList;
-  globPredicates = lib.concatMapStringsSep " " (g: "-find ${g}") fileGlobs;
+  globPredicates = lib.concatMapStringsSep " -or " (g: "-path \"./${lib.strings.removePrefix "/" g}\"") fileGlobs;
   copiedBins = runCommand name {} ''
     mkdir -p $out/bins
-    find ${lib.concatStringsSep " " processedList} ${globPredicates} ${fileFindPredicates}| while read -r filepath; do
-      cp "$filepath" $out/bins
+    for pkgPath in ${lib.concatStringsSep " " processedList}; do
+      cd $pkgPath
+      find . \( ${globPredicates} \) ${fileFindPredicates} | while read -r filename; do
+        fullPath="$pkgPath/$filename"
+        destPath="$out/bins/$(basename $filename)"
+        if [[ $(head -c 4 "$fullPath") != $'\x7fELF' ]]; then
+          continue
+        fi
+        if [[ -e "$destPath" ]]; then
+          echo "Uh oh: conflict for populating $destPath from $fullPath"
+          exit 1
+        fi
+        cp $fullPath $out/bins
+      done
     done
   '';
 in copiedBins