I have an engraving ball setup so it is easier for me to engrave as much as I can off of the rifle. If you have a setup for engraving then you will find the job easier to engrave off of the rifle.
There is evidence that many patch boxes were engraved on the rifle. You can tell this becasue you can see where the graver nicked the wood along the edges of the box where the engraved line ended at the edge of the box. There are also boxes that you find no signs that the box was engraved on the rifle. Some of these I feel certain were engraved off of the rifle. Can I prove that 100%, NO, so please no bad JU JU comments.
I also think that some of the better engraved boxes may have been done by an engraver and not the builder. Again this is just a guess on my part.
It is your rifle and you get to decide on how you want to engrave it, It is my opinion that you would be right either way.
There are many ways to hold done your work for engraving.
Thremoloc by GRS is a good product and as others have said it is reuseable. A word of caution if you decide to get some and use it. You have heat it in a microwave to soften the material to be able to form it to hold your work. I have had it to get hot spots in the material and it can and will blister the skin if you get it on you. I used to test the material with my thumb to see if it was soft engough to form, the last time I used it I happen to place my thumb in this hot spot, the material ahears very well hot and did so to my thumb. I could not get the material off untill it cooled, by that time it blistered my thumb.
I no longer test with my thumb or any other body part!
I will add that the reason that goo of any kind is used not only to hold the work but to keep the metal from bouncing as you are engraving. Any bounce in the meterial you are engraving will cause you problems and make the job much harder than it needs to be. This is also true if you engrave on the rifle. If you do not have good contact with the wood in your inlet you will get metal bounce or movement of the metal you are trying to engrave.