plop.p6

use NativeCall;

register-native-library('Gumbo', 'gumbo', v1); # could be register-native-library('Gumbo', ('gumbo', v1));

sub foo is native('Gumbo') { * }

plop2.p6

use NativeCall;

my LibHandle \GUMBO = register-native-library('gumbo', v1);

sub foo is native(GUMBO);

In my module I use

sub MyLibName {
    my $name = %*ENV<LIBLMDB> || 'liblmdb.so'; once note "Using $name"; $name;
}
constant LIB = &MyLibName;
...

And that results in:

[sog@monica p6-LMDB]$ perl6 -I lib t/01-basic.t 
1..49
ok 1 - The module can be use-d ok
Using liblmdb.so
ok 2 - Version LMDB 0.9.17: (November 30, 2015)
...
[sog@monica p6-LMDB]$ LIBLMDB="/home/sog/src/from-git/mdb/libraries/liblmdb/liblmdb.so" perl6 -I lib t/01-basic.t 
1..49
ok 1 - The module can be use-d ok
Using /home/sog/src/from-git/mdb/libraries/liblmdb/liblmdb.so
ok 2 - Version LMDB 0.9.70: (December 19, 2015)
...

Don't understand what are you trying to resolve.

Keep the native 'magic' (transforming foo into libfoo.so on unix, foo.dylib on os X...) while keeping a dynamic defined thing to give to NC.

Sure I can call guess_library_name myself in a sub and give it to the native trait, but it's an internal :)