Unfortunately I don't know anyone who customizes mail for others. Most of us who've done some tailoring have grudgingly done it just to make it fit better on ourselves, and probably couldn't be paid enough to do it for any other reason.
Having said that, there are some small-time mail vendors at some of the local faires. One of them from VARF is in my facebook friends list. I could shoot him a note and ask if he does that sort of thing. Or put you in contact with him.
Like Sir Brian, I'd love to see what you're talking about. Do you mean the armpit of the sleeve is too low on the torso, because of how wide the sleeve tube is? I've found that with the new GDFB haubergeon I got, when I clamp the 14th C. plate arms over it, it's slightly awkward because of how large the sleeves are at the armpit. I'm wondering if that's what you're running into? (EDIT: You posted while I was typing this. Yes, this is the problem.)
The armpits can be a little scary to work with, yes, but it doesn't need to be too bad. I made an exceedingly simple design on my aluminum hauberk. I never closed the seam there, just so I can raise my arms over my head without it pulling (this can be mitigated with a diamond-shaped expansion, or some-such), but if I hold my arms straight out, the seams line up real nicely. And all I did was leave some diagonal edges on both the sleeve and the torso.
Somewhere I have a diagram. I'll have to look for it.