The threaded plugs don't need to be deeper than 1/2".
There are two basic styles of hooked breech systems for flintlocks. Both probably originated in England, but could have also come from France or Germany originally.
The simplest has the hook on the the back of the threaded plug.
These were common on English and better American fowlers but can be adapted to rifles, too.
View attachment 86830
This one has a long tang suitable for a Hawken.
View attachment 86831
The actual threaded part of the plug is no different than that of a fixed flint plug and only needs to be about 1/2" long. The vent is drilled as a normal flint setup just in front of the face of the plug.
The other style is an overly simplified version of a Nock's Patent breech which excludes the anti-chamber that Nock developed. They have a deep powder chamber drilled in the face of the plug and into the "false" breech, as the English called them, or the patent breech as they are normally referred to today. The vent is drilled into the patent breech to intersect the powder chamber.
View attachment 86832
This is the same as above, just with a slant face where the patent breech joins the standing breech.
View attachment 86833
One of the problems with the patent breech style is that the powder chamber as made is often rather small in diameter and after a few shots, the fouling can build up enough for the powder to bridge off and not fill the powder chamber. If the barrel diameter and thread size is selected correctly, the diameter of the powder chamber can be opened up to just under the bore of the rifle, thus eliminating the powder bridging problem.