I know exactly what needs to be done with the OP so I can tell the seamstress directly, but I'm still worried as to how someone may react to a non-mainstream dress. My greatest fear is that I'll unwittingly take it to someone who will assume it's merely a costume and treat it as such.
I figure I should ask about scope of experience and if the proposed alteration would be too intimidating. Is there anything else I should check for? I'm mostly basing this off of meager experience I had with a family friend who was a seamstress (but is now unavailable).