I'm afraid that while I understand completely where you're coming from on this, there isn't a case I'd take to court without either (a) the client having been denied boarding or (b) some other objectively demonstrable adverse consequence as a result of having been put on that list. The law is pretty clear that until you can show damage in a tangible way, there is no standing to litigate.

The key thing here is that you aren't deprived of anything merely by being on a list of some sort. Once you have been, you have a deprivation of liberty without due process argument. A grayer area would be the case where someone somehow knows they're on the list and consequently doesn't travel. This one is arguable. But in any case, were such a person to come in and ask me to sue TSA over this, I'd recommend that they buy a cheap ticket somewhere and actually be denied boarding before proceeding. The deprivation of liberty argument is clearer that way and ultimately cheaper for the client, who would otherwise see their lawsuit held up while DOJ litigates the standing issue.