The short answer is that the police are likely acting within their power to seize the owner's firearms -- but it depends on the actual facts of the case and whether the proper procedure has been followed. Bear with me as I step through the legal analysis.
The question to ask here is whether 4th and 14th amendments restrict the government from seizing these firearms and if so, whether the government actor(s) followed the correct procedure in effecting such seizure.
The 4th amendment states:
"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
This restriction on power is applied to the states through the due process clause of the 14th amendment which states:
"[N]or shall any State deprive any person of life, liberty, or property, without due process of law[.]"
In essence, these constitutional provisions check state and federal governments from seizing any property without proper procedure and substance in executing such seizure -- referred to as "procedural due process" and "substantive due process" respectively.
Balance this with the Connecticut statute that the police are/will likely claim grants them the power exercised -- Conn. Gen. Stat. § 29-38cstates in relevant part:
"Upon complaint on oath by any state's attorney or assistant state's attorney or by any two police officers, to any judge of the Superior Court, that such state's attorney or police officers have probable cause to believe that (1) a person poses a risk of imminent personal injury to himself or herself or to other individuals, (2) such person possesses one or more firearms, and (3) such firearm or firearms are within or upon any place, thing or person, such judge may issue a warrant commanding a proper officer to enter into or upon such place or thing, search the same or the person and take into such officer's custody any and all firearms."
I am not opining on the (federal) constitutionality of this statute but notice that it provides a procedure for seizing firearms by requiring that a warrant be issued by a judge. This is the procedural due process. Moreover, the statute essentially requires that the authority act under "probable cause" (which is required when making any arrest or seizure under the 4th/14th amendments) that the owner is a risk and goes on to state, in portions not quoted above, that such warrant shall only issue with the support of specific affidavits sworn by the complaintant(s). This is the substantive due process.
In conclusion, if the officers followed the procedure outlined in this statute and a judge ordered a warrant for seizure after considering evidence, then the seizure was likely proper. Is so, then there is no way for the (former) owner to go around seizure by executing some type of step transaction through an FFL (as opposed to purchasing the firearms from a vendor).
If we are talking about pistols here, then there is no way to regain them or the right to possess pistols without going through a legal appeal or reapply for a license. Since the State can seize and cancel a license to carry a pistol (which is legal - i.e. constitutional), it can effectively revoke the owner's ability to possess these firearms because it is illegal to do so without a license.
P.S. - For the legal beagles out there, we know that the 4th amendment is fully incorporated to the states through the 14th amendment from cases like Ker v. California, 374 U.S. 23 (1963), Aguilar v. Texas, 378 U.S. 108 (1964), and their progeny. On a personal note, I believe that selective incorporation is a bastardization of the clear meaning and intent of the privileges and immunities clause of the 14th amendment. We are likely stuck with it, however, as the court's transcript in the McDonald case indicates their unwillingness to revive it as Slaughterhouse is now too long accepted to reverse.