Ask on the website of the International Society of Glass
Beadmakers: http://www.isgb.org/index.shtml Click on the "ISGB
Forum" link on the welcome page and register as a new user so that
you'll be able to post your request in the category concerning
beadmaking. It would help if you could post a copy of your sketch
so that artists who might be interested could check it out. I
think that the 1/4" diameter opening could be achieved. Some of
the beadmakers have used tubular steel mandrels as large as 1/2"
Responding to this, even though I can’t find the first post about
ISGB is a great forum. But it’s a non-profit forum, and there are a
lot of rules about selling things or making money via posts there.
Since I don’t know the exact query, nor all the details of the rules
on isgb (it seems kind of random sometimes), I can’t comment as to
whether or not this question will be removed, just stating that
there is a possibility of it happening.
Another option is www.wetcanvas.com. The rules about money are a
little more lax there, and more things fly, even though their policy
is vague too. I think more glass artists who would take a comission
for custom work are on wetcanvas. Plus, it’s a lot easier to see
their work, since pictures can be hosted on the site.
I do know that glass beadmakers can do work on larger mandrels.
Quite a few people make glass rings, so the only difficulty is
finding the supplier for the mandrel. I think one of my catalogs has
mandrels up to 1/2" listed in it.