What i do is the folowing :
When an user connects to the site, i swpawn a gen_server reprensenting the user. Then, the gen server registers itself in gproc as {n,l, {user, UserName}}. (It can register properties like {p,l, {chat, ChannelID}} to listen to chat channels. (see gproc pub/sub))
Ok so now the user websocket connection starts the cowboy handler (i use Bullet). The handlers asks gproc the pid() of the user's gen_server and registrers itself as a receiver of messages. So now, when the user gen_server receives messages, it redirects them to the websocket handler.
When the websocket connexion ends, the handler uregister from the user gen_server, so the user gen_server will keep messages until the next connection, or the next timeout. At the timeout, you can simply terminate the server (messages will be lost but it is ok).
See : (not tested)
-module(user_chat).
-record(state, {mailbox,receiver=undefined}).
-export([start_link/1,set_receiver/1,unset_receiver/1]).
%% API
start_link(UserID) ->
gen_server:start_link(?MODULE,[UserID],[]).
set_receiver(UserID) ->
set_receiver(UserID,self()).
unset_receiver(UserID) ->
%% Just set the receiver to undefined
set_receiver(UserID,undefined).
set_receiver(UserID, ReceiverPid) ->
UserPid = gproc:where({n,l,UserID}),
gen_server:call(UserPid,{set_receiver,ReceiverPid}).
%% Gen server internals
init([UserID]) ->
gproc:reg({n,l,{user,UserID}}),
{ok,#state{mailbox=[]}}.
handle_call({set_receiver,ReceiverPid},_From,#state{mailbox=MB}=State) ->
NewMB = check_send(MB,State),
{reply,ok,State#state{receiver=ReceiverPid,mailbox=NewMB}}.
handle_info({chat_msg,Message},#state{mailbox=MB}=State) ->
NewMB = check_send([Message|MB],State),
{noreply, State#state{mailbox=NewMB}}.
%% Mailbox empty
check_send([],_) -> [];
%% Receiver undefined, keep messages
check_send(Mailbox,#state{receiver=undefined}) -> Mailbox
%% Receiver is a pid
check_send(Mailbox,#state{receiver=Receiver}) when is_pid(Receiver) ->
%% Send all messages
Receiver ! {chat_messages,Mailbox},
%% Then return empty mailbox
[].