It's not the "effort" that makes roasting heirlooms not worth it--it's that, in my opinion, it's a waste to use them that way. The only time I'd use heirlooms in sauce is if they're getting past their prime for eating raw.
As for your other question, while you may be able to find some juice someplace (try asking any beef place that you like--I'm sure they'd be happy to give you some if you ask nicely), you can also make an approximation by getting a few beef bones and making a quick and easy stock (roast on high in the oven or brown on the stove with some onions and a bit of olive oil until they are well-carmelized, then add water and your herb(s) of choice and roast or simmer on the stove top until you reduce by 1/3. Season with S&P and you have stock. Pretty easy to do. Not, obviously, as easy as buying from a restaurant
"Knowledge is knowing a tomato is a fruit; wisdom is not putting it in a fruit salad." Miles Kington