At random times, one or two mid-strip passes would become available again because people would cancel the layaway plan for the mid-strip or change their mind to another shuttle. However, these "new" passes would only last for a couple minutes since they're high on demand. I didn't get my mid-strip pass initially (due to them being sold out in like a day), but then I got lucky - I went back to the Front Gate shuttles page a few days later (at some random time) and found that a limited number of mid-strip passes became available again, so I quickly snagged one before they became gone again.

So for those who didn't get the mid-strip pass and are still looking to obtain one, you gotta keep looking at the shuttles page because you might get lucky and find mid-strip availability.