> "And what if something turns up which would retroactively justify it?"

US constitutional law prohibits the introduction of evidence obtained illegally.

https://en.wikipedia.org/wiki/Exclusionary_rule ("Exclusionary rule")

There's no "retroactive" exception. The core point of this rule is to deter police from intentionally violating people's rights, under the expectation that what they find will, "retroactively", vindicate them. Won't work.

> What recourse would an American have against a punitive search?

None. The endless videos, from better-years-gone-by of people refusing to answer questions at the border then having drug dogs run all over their car to scratch it up was my first exposure to federal agents acting maliciously.

You can attempt to sue for damages, but the suit is likely to be dismissed because law enforcement and legal adjudication are tightly coupled and very friendly in ways that subvert the proper functioning of justice. More likely you'd just invite more harassment for daring to attempt recourse at all.