I don't have any personal pics for you, but I have used diamond tipped dremel bits to grind detents into a few different things before. One was a specialty 1/4" drive adapter that I ordered off of Ebay and it came without detents. I can't remember what the other things were, but I have done it at least 3-4 times with 1/4" and 3/8" drive stuff. It works just fine and the diamond bits last a long time as long as you keep them wet, cool, and go slow. I use a few drops of WD40 (from a small dropper bottle) to keep the bit cool as it spins, but other things would probably work too. If I remember correctly I did put the 1/8" dremel bits into my rotozip for more torque instead of using my dremel, but it should still work just fine in a dremel, it will just take longer.
If you can find on Ebay etc. a set of ball head diamond bits like the one in my pic, those work pretty well and will be the easiest way to make a nice looking detent. Just estimate the ideal ball diameter by looking at other existing tool detents of the same drive size. Realize though that I have only done a few tools, and for the tools I was modifying I only needed to do one or two detents per square hole. You might want to do all 4 sides for a crowfoot wrench set so you can quickly flip the crowfoot wrench around and always have ball detent engagement. That will take longer.
All this being said, none of my crowfoot or flare crowfoot wrenches have ball detents, and I have never missed them. Spring ball tension on extensions is enough to hold them on there just fine for what I do. There is still a good amount of resistance to them pulling off. I almost prefer them to not have them, so you might want to just try using the wrenches without modifying them and see if it really is an issue for you to begin with. It will take a while to do 4 nice detents in every crowfoot wrench no matter how you do it.
In as far as a time estimation, it might differ some depending on the quality of the diamond ball bit, but I bought a set of pretty cheap ones and they worked well for me. I think the bits were about $2-3 each or so and I am still using the first one. You might go through 2-3 bits to do as many detents as you want to do. I would estimate about 1-2 minutes per detent for a nice looking well done detent, so plan an hour or so per set of crowfoot wrenches since you would ideally want to do 4 detents per wrench.
