I remember reading this technique in Susan Van Leuven's Woodstrip Canoe Building book.

Take your cool, mixed, thickened epoxy and put it in a heavy weight ziplock bag. Remove the excess air, snip off a corner, & apply a bead of epoxy to the joint. This is exactly like using a pastry bag to decorate a cake. This is a very neat and fast way to lay down a bead & smooth out a fillet without too much hassle. After a length of bead is put down, use your finger, or other implement to create the fillet in the joint.

As far as mixing the epoxy to get a longer pot life, I would add the thickening material to one of the components. Cool each component. Mix the two together in a wide container.

Just a thought that could speed things up enough to get some working time out of the epoxy.