I wasn't looking for a travel bra, but happened to see this in Costco and it struck me as the ideal travel bra, at least for someone like me. I'm not trying to advertise. Just noting that Filena "Side Smoothing Bra," seamless, tagless, wire-free, style # 74428 is comfortable for all day wear and would probably be great on a long flight.
It has removable modesty pads, so if you wash it out and need it to dry overnight you can slide the pads out and just wash the bra part. EDIT: I tried machine wash, cold, in a "delicates" bag and found that these pads shift during the spin cycle. Each pad is a contoured equilateral triangle (i.e., there's no L-R or inside-out difference), so it's fairly easy to pull them out and insert them in the proper place, but if you ignored this until dry you'd have a badly crumpled up set of pads. I will hand wash in future. Obviously, they're not to go in the dryer.
Drawback: It gives a rather smooshed/flat silhouette like a sports bra. OTOH if a minimizer is what you like then this would really do the trick. It comes in S-M-L-XL and I found it runs slightly small, so if you're between sizes go with the larger.
At Costco it was about $20 for a pack of two, one beige and one black.