Seems like it would be more trouble than it's worth.  If you put enough thickened epoxy into the cartridge to make it convenient the contents will heat up and harden before you get a chance to use it all.  Clean fillets are just one of the steps in the process that take a little extra preparation, effort, time and practice.  In my experience, the best advice for good looking fillets is to remove the stitches...again, an extra step but the extra steps are usually well worth it in the end.

Chris

I have to agree with chris, I tried this and the epoxy started to go off before I could get started and I went back to the tried and true method gob it on and spread it I used a ice cream stick. Good luck

Larry

I had the same problem when I built a canoe a few years ago. My solution was a mini caulking gun: a syringe. I went to the local Tractor Supply Company (TSC) and bought a bag full of small syringes without needles. They come in a variety of sizes.  There easy to fill up, use, and fill up again. There cheap enough to through away when your done.
Have fun!
StingyRog

I have had success using System Three slow cure epoxy mixed fairly stiffly with wood flour.  If you are well prepared, it is possible to get multiple uses out of a single West System tube (the empty ones that you fill yourself). 

I would mix a batch large enough to fill the tube twice, in a big tray type container, then spread out the mix across the tray to have as much surface area as possible to keep it 'cool'.  Then I would fill the tube, make the fillets, and re-fill the tube and make more fillets.  Then I cleaned out the tube with some white vinegar on a wad of paper towels.  This way I could re-use the same tube again and again.

This was during early spring and the temperature in my shop was only about 55 or 60 degrees F.  It would be much more challenging at higher temperatures, maybe not even possible on some summer days.

Ron http://jimmyskiff.blogspot.com/2007/03/installing-furniture.html

Thanks for your comments, especially Ron. I was hoping to hear from somebody who had actually used these tubes. I expect it will take less than ten minutes to empty a tube and would be surprised if the epoxy kicked off in that short time. I do use syringes for tacking pieces together before removing the stitches, but they are too small for the thick mixture used for fillets. You can follow my progress on my blog site, but I probably won't begin construction until mid-August, as summers are very busy times here on Cape Cod. -Wes
http://twofootartist.com/merganser-construction-notes/