Gee, I know I was only kidding and I know you were just going along with the joke, but your idea might actually work. Make it so only a planet or system with say 40B people can build the facility and make it so you can choose the number of ships you can currently make (so you could "summon 1 ship" knowing you can esily build it later, or 20 ships and hope you can get them done in "time") If for example you only have 5 SY in the system multiply it by the number of turns to the future you call to and get the maximum you can summon.
Edit: mind you theres the Bill & Ted factor
"You forget, only the ones who win can come from the future"
