When I add code to the .bs file, it is executed, so I used that to modify @DynaLoader::dl_library_path.
What about adding that code in the actual .pm file you created (Rmath.pm?) like:
package Rmath;
...
our @ISA = qw( ... DynaLoader ... );
require DynaLoader;
push @DynaLoader::dl_library_path, 'my-nonstandard-lib-path';
print "XXX=$_\n" for @DynaLoader::dl_library_path;
bootstrap <Yourmodule> $VERSION;
$dl_debug : I meant doing export LD_DEBUG=all in your terminal/shell/prompt before testing your program (on that same terminal).
There are two very remote possibilities: 1) the dylib extension is not tried at all in enumerating possible library names (unlikely because it finds it in standard lib location), 2) the spaces in the library path (in OSX hmm that's not likely).
ps. the reason I suggested this reverse-migration is to investigate DynaLoader's behaviour without the front-man.