A couple of thought on this topic if I may
On deforming threads: Irrespective of metal or plastic... I've used El Grandes for years, posting, with no deterioration. Remember every time you close the pen you tend to restore the threads, right? At least I think this may contribute to the longevity of the threads.
On whether customers care about posting or not: It *REALLY* depends on where you sell! Customers at craft shows have completely different values than customers at a fountain pen show. Even so, even at fountain pen shows, I do sell a lot of heavy, non-posting pens. Pens I could never use, but as long as folks buy them... There is no right or wrong, important ot unimportant. You'll *ALWAYS* have some customers that care about certain issues (including postability) strongly, and there is not a single right pen. Long live diversity!
I have made posting pens along the lines shown here, and don't right now. I simply am not thrilled with the look. This is my opinion, not necessarily any customer's
I really don't like the stepped main barrel. I really find it less objectionable when there is a material change, or an offsetting band. I also have a beef with poorly fitting posted caps, as in the El Grande or Churchill. It looks to me like PSI got that right with thier new Olympian - the cap posts very nicely. So, if you want to make this sort of posting cap, it is REALLY important to get the dimensions of that main tube end just right so the posted cap looks integrated. I cannot tell from the photos above whether you achieved that, and can achieve it consistently. Maybe it is not something you care about strongly, and that's OK with me
The best posting caps IMO are on commercial pens like Pelikan. Or Montblanc I suppose. It simply fits right! The friction fit is effortless, and the holding power very good. Because of a large overlap it does not need as much friction. It's completely different type of fit from what you are trying to achieve, duplicating El Grande or Churchill. I am not trying to persuade you to pursue something different, but if a great posting cap is your goal, check these out. The problem we have with these kit designs is that the total thickness cap threads plus plastic plus brass tube plus outer material is quite substantial. On a Pelikan, the cap is very thin. There is more overlap to the body. So the overall posted pen is shorter and IMO better balanced. The reason I am pointing this out is that a lot of people seem to be interested in cutting their own multi-lead threads suddenly, and that is a key ingredient in getting such a fit. BTW... you don't need the multi-lead taps to experiment on such designs. You can get a 1/2"x32TPI tap and die as stock parts from an industrial supply house (like MSC) for around $30-40 combined... Anyway, I hope some folks go down that path, lots of potential for great pens there! All kit pens have the threads at the cap opening. Pushing the threads further up in the cap opens up lots more possibilities!
While I am musing about postable caps, the issue of threaded posting comes to mind. Like the Baron. Again because of the wall thickness of the cap there is a huge step. But, additionally, the clip often ends up in the wrong position... with a multi-start thread, the clip often ends up in a position that interferes with the writer's hand. So, a shorter single-start thread may be a good answer to this issue. A decent friction fit does not have these issues. You do have to worry about the contact point though... if it is the rim of the cap, you *WILL* split the cap sooner or later. This is why a lot of pens incorporate a metal band there to absorb the stress. Another challenge
Just some food for thought. Glad to see people interested in this!