With some input from Jon I've managed to reduce this back into a macro rather than a fun and a macro calling a fun. He also suggested that on eagain I sleep a small amount of time so as to allow other work to catch up a bit.

src/async_nif.hrl

@@ -21,28 +21,26 @@
 %%
 %% -------------------------------------------------------------------
 
--spec async_nif_enqueue(function(), [term()]) -> term() | {error, term()}.
+-define(ASYNC_NIF_CALL(Fun, Args),
-async_nif_enqueue(F, A) ->
+	F = fun(F, A, R, T) ->
-    R = erlang:make_ref(),
+		    case erlang:apply(F, [R|A]) of
-    case erlang:apply(F, [R|A]) of
+			{ok, enqueued} ->
-        {ok, enqueued} ->
+			    receive
-            receive
+				{R, {error, shutdown}=Error} ->
-                {R, {error, shutdown}=Error} ->
+				    %% Work unit was queued, but not executed.
-                    %% Work unit was queued, but not executed.
+				    Error;
-                    Error;
+				{R, {error, _Reason}=Error} ->
-                {R, {error, _Reason}=Error} ->
+				    %% Work unit returned an error.
-                    %% Work unit returned an error.
+				    Error;
-                    Error;
+				{R, Reply} ->
-                {R, Reply} ->
+				    Reply
-                    Reply
+			    end;
-            end;
+			{error, eagain} ->
-        {error, eagain} ->
+			    SleepyTime = min(30, (T+1)*2),
-            %% Work unit was not queued, try again.
+			    timer:sleep(SleepyTime),
-            async_nif_enqueue(F, A);
+			    F(F, A, R, SleepyTime);
-        %{error, enomem} ->
+			Other ->
-        %{error, shutdown} ->
+			    Other
-        Other ->
+		    end
-            Other
+	    end,
-    end.
+	F(Fun, Args, erlang:make_ref(), 1)).
-
--define(ASYNC_NIF_CALL(Fun, Args), async_nif_enqueue(Fun, Args)).