Draco: I can see the potential for quality control problems with just trying to do the ends. They soak up the oil primarily through osmosis, i.e., along the channels running lengthwise in the wood. So even dipping them in six inches of oil, they might draw in 8 inches, or 10 or more, or less than 6 inches, depending on how long you leave them in the oil. Timing becomes critical.
Soaking just the end doesn't really do much for FOC. Based on my measurements of several hundred shafts over the past few years, soaking 6 inches of the end of the shaft would only add about 15 grains, not enough to affect FOC much
Snag: Just pulled my shafts out of the tube yesterday after soaking for about six days with a little pressure to start. After a day of drying, the 5/16 shafts gained an average of 48.5 grains, the tapered 11/32 shafts gained an average of 80 grains. They don't seem to be bleeding, probably because I didn't use very much pressure in the tube. That's about 10-20 grains higher than I normally get without pressure. The fact that I was using fresh oil might also have helped.
The darker stain (Minwax ebony)I added to the natural Watco oil didn't take real well, but it did darken them some, but not as much as when I use the stain on bare shafts.
I'll be interested in how your Doug firs react to the oil. As I mentioned earlier, for me, they picked up almost no oil at all -- only 10-20 grains for 11/32 shafts.