Well .... I would say NO, the Size S ( 0.348" ) will NOT work in place of a size 11/32 ( 0.3438" ) ..... but I could be wrong.
If you measure brand new drill bits, right out of the package, they are about 0.002" below the stated size ... that might be a helping factor.
However, I did not know that a Size S barrel trimmer is a thing you can buy off the shelf. . Is it ?
I have never used a barrel trimmer. . I do my end trimming ... all trimming to accurate length ... on the lathe.
You have to do it extremely carefully, mind !! . And it helps to have a decent head stock chuck in which to mount your blank for trimming.
Do you have pin jaws for your head stock chuck ? . Check them out. . For starters, at PSI website, altho' that's not a recommendation
What I have NOT told you is that I do all my pen making operations, including end trimming, on a metal working lathe.
So, consequently, I really do not know what I am talking about when it comes to wood turning lathes.
But something similar to what I am suggesting MUST be possible on a wood lathe ... avoiding the purchase of ANY end mills.
Maybe someone else will chirp in to help you ..... maybe I can learn something from this also ... I'll follow this thread.
BTW ... just re-reading your post ... you say :-
"I would think that 0.022 larger would not really be noticable." . How do you get that number 0.022 ? . Possible mistake ?
You would know that the 11/32 sizing refers to the hole drilled in the blank to accept the brass tube OD .... oversize from the tube.
And also, that the barrel trimmer drill bit has to fit the INTERNAL diameter of the brass tube ... Just trying to clarify if necessary.
And here is another BTW .... I did a search .... there does seem to be, altho' out of stock, an 11/32 end mill shaft (fits ID of 11/32 tube)
Whiteside 11/32" Pen Mill Pilot Shaft : Amazon.ca: Tools & Home Improvement
www.amazon.ca