In case the thread/topic proves relevant to someone's search in the future, we elected to give the headspace a little time in the barrel under the assumption that the CO2 will occupy the space, then dissipate through the oak and airlock.
We've since brewed a 10 gallon batch of the same beer. Half of that batch is currently fermenting using the WL Belgian Sour Mix as the primary yeast. That one is definitely a first for us, but we have time and it will be added to the barrel that already has about a month of a head start on bugs freshly added to the secondary at the time of transfer. Somewhat surprising, we have visible attenuation at about 24 hours.
If anything else relevant happens during the course of this experiment, I'll update the post for the (hopeful) edification of others' adventures.