Sell the Sub for a good down payment, charge the rest on a credit card, then do like Larry said, work a few weekends for some overtime to pay for the rest.
Actually, think of it not as what you have to get rid of to get the Luke, but which would get the least love after the Luke arrives. Does it make more sense to lose the HSS trem, since you'd be picking up an HSS trem, or is that what you need for your playing style and then go back and forth between the EMGs and passives?
The only thing not replaceable is the color of the SS, if you can't live without that, then it would have to be one of the others. The lead time on a custom order is also great for selling the SUB for the deposit and figuring out a way to come up with the balance over several months.
I'm at the one out for one in stage myself, and got a line on a used that I'm debating, but I think I'll wait until there is some BFR AL info before making that decision