Craig
I saw an approach somewhere several years ago that I used once or twice. It does work most of the time, but since the idea is to save a transmission that otherwise would have to be scrapped, the fact that it sometimes fails is probably OK.
It requires that you make a jig (of course). Drill a 1/4" hole in a scrap hardwood blank about 2" long, and then carefully cut a saw kerf longitudinally about 2/3 of the way through the blank, making sure that the kerf aligns with the axis of the hole. The jig then becomes a soft collet to grip the transmission without crushing it.
Find two flat washers that fits over the transmission, and slip them down to the end where the transmission enters the pen body. Then, put the transmission into the collet, and grip it in a bench vise so that the split portion of the collet is squeezed by the vise. Slide the transmission down into the collet and the end of the blank where the transmission has been inserted, leaving 1/8" or less free space, and clamp down on the vise to firmly grip the transmission.
Then, slip a flat-blade screwdriver between the two washers and gently twist. Twisting the screwdrive forces the washers to separate. Since the transmission is gripped by the grooved block held in the vise, the only way the washers can move apart is for the upper washer push the pen body off the end of the transmission.