Real simple.
YES, you can have a threaded barrel in CALIFORNIA! IN the pistol. But to do so legally, you must ensure the cap is 'permanently' adhered to the barrel to where standard common hand tools will not remove it.
Now the question comes down to, what constitutes 'permanent'. Research P22 California DOJ. They allowed S/W, Walther to use either JB Weld/Loctite to remedy that situation-no welding. Of the 7 (SEVEN) LEOs I have spoken with that I know, AND 5 FFLs that I deal with, 3 ranges, they ALL have said the same thing. Welding is not necessary nor is pinning and they have never witnessed someone with a cap on their barrel inspected. Red Loctite is all that is needed. The closest the DOJ has come in regards to this issue is the mentioned Walther P22 problem. Also the new cap did not have wrench cuts on it. To be on the safe side, I recommend have it pinned. That is what mine is and I shoot next to LEOs all of the time, they fired my pistol often to check it out. Never an issue other than "that is bad ass!" I also know a few with just barrel caps which are Loctite'd on (HK,s, etc). Never an issue. Now if your at the range or elsewhere and LEO inspects your pistol and can remove the cap with his/her hands-your in trouble. But bottom line, they are more concerned with mag capacity than threaded barrels.
AND YES, you can still clean the barrel and slide with this setup. Easy! If your at the point where you need a new barrel, have it un-pinned or cut it and install a new one.
I can easily take anyone's pistol here with a standard barrel and turn it into a threaded barrel easier than unpinning a threaded cap on a threaded barrel.
For SSE purposes, FFLs appear to offer a few choices. One I know sold them as 'parts', not assembled. I brought my barrel from my standard plain jane FNX-45. Others would not touch it period.