Use per-user (as opposed to per-project) rebar plugins in ~/.rebar/

config

%% -*- erlang -*-
%% As an example, let's make the dialyzer plugin available regardless of
%% whether the project in question uses it:
{plugins, [rebar_dialyzer_plugin]}.
%% config.script uses this new variable to find it:
{plugin_home, ["/Users/magnus/.rebar"]}.

config.script

%% -*- erlang -*-
%%
%% For any plugin in the plugins directive in ~/.rebar/config that is
%% not already in the code path, add ~/.rebar/PLUGIN/ebin to the code
%% path and assert that we can find it.
PluginHome = proplists:get_value(plugin_home, CONFIG),
Plugins = proplists:get_value(plugins, CONFIG, []),
lists:foreach(
  fun(Plugin) ->
	  code:which(Plugin) =:= non_existing andalso
	      code:add_path(filename:join([PluginHome, atom_to_list(Plugin), "ebin"])) andalso
	      (is_list(code:which(Plugin)) orelse error({plugin_not_found, Plugin}))
  end, Plugins),
CONFIG.