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

@@ -21,9 +21,8 @@
 %%
 %% -------------------------------------------------------------------
 
--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
 			{ok, enqueued} ->
 			    receive
@@ -37,12 +36,11 @@ async_nif_enqueue(F, A) ->
 				    Reply
 			    end;
 			{error, eagain} ->
-            %% Work unit was not queued, try again.
+			    SleepyTime = min(30, (T+1)*2),
-            async_nif_enqueue(F, A);
+			    timer:sleep(SleepyTime),
-        %{error, enomem} ->
+			    F(F, A, R, SleepyTime);
-        %{error, shutdown} ->
 			Other ->
 			    Other
-    end.
+		    end
-
+	    end,
--define(ASYNC_NIF_CALL(Fun, Args), async_nif_enqueue(Fun, Args)).
+	F(Fun, Args, erlang:make_ref(), 1)).