Hello AK1.

I hate to rain on your parade, but...
If you book your revenue ticket in J space, their has to be C space available for the Companion Ticket. It is taken out of C, not J, eben though you are buying the rev seat in J.
Which brings up the next point, if you were to buy Y or less fares with the hope of upgrading at time of booking, you are S.O.L., you will have to hope for an op up at the gate or ckin.

Having said all this, I have successfully used the DC MLL Comp Tkt on this route. I don't want to discourage you , but give you a headsup.

Good luck