Switching to grain-free dog food made a shocking difference in my arthritic dog's mobility. I don't know if joint supplements would make any difference in light of the metacam, but Dasuquin was by far the most effective for my arthritic dog and it significantly improved her comfort level and mobility.
My apologies for the novel - the only difference below between this dog and my last dog who had far more serious arthritis was the moving onto a new generation drug of choice. Otherwise the same programme and I managed my last dog through 6 years of severe arthritis and he enjoyed most days.
My (not quite) 6 year old BC has arthritis in her front right wrist and my vet has put us onto "Trioxcil" (sp?) - a long lasting anti-inflamatory . As in one pill a month and is a lot kinder on the stomach than either metacam or rimadyl or tramadol etc! We are on month 2 and it really seems to be working. This is combined with a daily low dose of Synoquin (you will need to google this as it may have a different brand name in the US) and monthly catrophen injections (you will need to google this as it may have a different brand name in the US - I think the latter is known as adequan in US). This may help your girl.
We are also doing physio (every 6 weeks from a licenced animal physio) and daily stretches - which she loves as there is a lot of strokes from Mum ).
Also look for exercises to assist in hind-end stregthening. As we are in our summer, I am also doing a lot of swimming in our local river which has quite a strong current - and I mean swimming not walking on the bottom. I go in as well, and we walk/swim upstream together for 5 minutes at a time - for a total of about 30 minutes after building up to that over a period of about 4 weeks. This is a sort of physio/non-weight bearing exercise - and at temps in the high 20s/low 30s over a lot of Christmas/New Year, it has been quite pleasant in the river. (And I have been doing the same with the other three dogs in our household lately and it is awesome for getting the labs down below ideal weight really quickly - once they get the hang of "going for a swim" they seem to enjoy it.) Is this a possibility for you - obviously not in winter but in your next summer. I have met a lot of older/old dogs at our river this summer - and most seem to be doing the same as me.
My BC is also toy obsessed and I have found that using "send-aways" as my goto exercise of choice has helped stregthen her back and back-end to become stronger. No explosive running out as I let fly with the ball thrower but still a lot of fun as well as re-inforcing her obedience exercises.
She is doing so well that we are back doing agility (at mini height in training - she is a maxi dog) to see if she is stable enough to continue with agility (at a lesser rate) - at the vets recommendation. So far, so good.