Anybody used a in line transmission filter? Do they work or not?

Yes, I installed a magnetic disposable unit mad by an Aussie company sold by NAPA years ago on my 727 in my truck.The specs looked great (micron size)and I reasoned I could double or triple the length of time for the in pan Dacron filter, write a date on the in line, and toss it every year based on mileage.
Zero issues.They alaso make something very similar for power steering on the return line.

in my 2004 wix master catalog, there was a magnetic in line transmission filter listed: pn 58964. the description states it as a double-action, full flow filter down to 25 micron size particles. also states if filter becomes plugged, it resumes full flow thru a pressure relief valve. it handles up to 200lbs pressure, 350*f, and removes over 95% of all magnetic impurities introduced into the trans fluid. to be installed in the rear line on 904/727 transmissions. can also be used on power steering return[low pressure side] lines. i'm not sure if the pn is current, or has been superseded. hope this helps.