Rather than the ACLU matter, I suggest you look into the separate lawsuit filed by Oregon Attorney General (AG) Ellen Rosenblum. That seems to be more based on the concerns you identified, e.g. the statement by Mark Pettibone asserting he was arrested without probable cause. See also this NBC report. While the Oregon AG won't necessarily be interested in your out-of-state input, if you know someone more directly involved in this, the office of the AG's general phone number is publicly available.
Also separately, U.S. Attorney Billy J. Williams, the Department of Justice's chief law enforcement official in Oregon, called for an inspector general investigation into DHS personnel over reports of two protesters being detained without probable cause. UPDATE: DOJ Inspector General (IG) Michael Horowitz said today his office was opening an investigation.
While neither of these actions necessarily provide you a way to "show support", they do provide indications that the allegations are being taken seriously. Of the two approaches listed above, I personally give the IG investigation a better long-term chance of identifying meaningful evidence of any civil rights violations, if they occurred. The system has its checks and balances, but they aren't necessarily responsive to hyperbolic claims, or statements of support vs opposition from individuals who wouldn't have standing in court.
PS: Being arrested and then released without charges isn't too surprising for protests as massive and violent as Portland has had. Law enforcement officers make arrests based on "reasonable suspicion", but charging decisions (in federal cases) are made by lawyers working for the U.S. Attorney, who care more about sufficient evidence to get a conviction. There's nothing that requires federal law enforcement officers to make arrests only on federal property, nor to drive marked vehicles.
PPS: There's a particular Oregon law that may be causing some confusion here. That law permits federal agents/officers to make arrests based on Oregon state laws, but it adds requirements around arresting officer identification that apply only if the arrest is made based on state (not federal) law. People who may want to politicize the issue seem to skip over that detail.
Also, the judge in the Oregon AG's lawsuit just denied the state's motion for a restraining order. Addressing some of the issues raised, the court said:
The State has presented just one example of an arrest without probable cause and one example of an unreasonable seizure. That is the sum total of the evidence before me that underpins the legal injuries the State asserts in its brief,” the judge wrote. “In both instances of a federal seizure it is either admitted or clearly visible that the agents’ uniforms say ‘Police.'
The injury the state asserts is entirely conjectural. First, the state candidly admits that it does not have a shred of evidence that counter-protesters have ever, anywhere, kidnapped a protester or anyone associated with protests,” he wrote. “Second, the asserted interest rests on an utterly implausible inference. The State’s reasoning is that counter-protesters, once they learn of seizures of protesters by federal agents, will dress up like police and go out on private missions to kidnap protesters. This despite the fact that such kidnappings are Measure 11 felonies in Oregon, punishable by mandatory minimum sentences of up to 70-90 years in prison.