When I've done a starter batch like you're talking about, I've just primaried the starter batch for a week or so and then racked it to secondary. Then, once the biggun is chilled, pour it right into the primary with the starter yeast cake in the bottom. You probably want to think about a blowoff setup as that biggun fermentation is most likely gonna take off like a rocket.
The other way I've done it is to go ahead and rack the starter to secondary and "wash" the yeast cake from the primary. Depending on the size of the biggun, you may have enough washed yeast slurry for it (check Mr.Malty). If it's not gonna be enough, you probably gotta stick with the first method.