Booster fans can be equiped to run ,whenever the central system fan runs,that would be my recommendation.
http://www.fantech.net for more info. on fans.
Mastic is one way to seal all the seams .joints and connections,difficult to do on an existing home.
You have already mentioned the "thight" space the ducts are in,so I doubt they could be completely sealed.If you hire someone to seal,insist on a "duct blaster" test before and after to see the results.
For existing homes go here http://www.aeroseal.com ,although there's not a dealer in your area at this time.