Rfing the Radian on planes can be problematic, and while I'm not concerned in the least about ffing a 1yo on a plane (as long as they rf in all vehicles) sitting that upright for that long with a newly applied cast might be an awful lot to ask of a barely 1yo. Then you also have the issue of getting the harness tight when one side of his body is sitting lower than the other.
(you shouldn't be able to pinch any slack in a properly tightened harness, BTW)
In general, the type of casts used in serial casting shouldn't cause seating problems, unless he has significant twisting and they have to cast his hips abducted. It
will magnify how unevenly the two sides of the harness tighten down though, since you lose the ability to "squish" the high side a little in order to make the low side harness tight enough once the cast is on. Due to this, I would suggest getting a seat with a continuous harness, meaning the hip straps don't end on a bracket under the seat like on your Britax, but rather are one long strap that goes down from one side, all the way under the shell and up the other side. This lets you pull slack from one side of the harness to the other and have uneven straps as needed. You may need this feature the entire time your child is casted, not just for traveling.
If you can post back with a height, weight, and shirt size of your little one, we can recommend some seats with continuous harnesses to check out.
Oh! Knowing your budget helps too