If the Warnock guillemets are right, and this is just for one or a few documents, how about simply replacing Arno with Warnock for that character? That's be super cheap.

Yes, as forrest said, just swapping them out would be the cheapest. However, I'll point out that the weight is slightly different, and the terminal ending in the fonts (including the guillemets) is a little different. Swapping them out would work perfectly fine, though. It would definitely be best to do this directly to the font. If you can't afford (or figure out how to use) the software, I could easily do it for you, either for free or a very reasonable amount (it's only a few characters).

Email me if you're interested.
artistenator at gmail dot com

Also, if it's only going to be a couple pages, you'd be just as well to do a search and replace.

Does their EULA allow third party modifications?

Using the Warnock guillemets would be an option, yes, but I would prefer a custom font. And thanks for the offer, brianskywalker, maybe I'll come back to you.

@frode frank: Well kinda, see this thread.

Faq, yes. I guess you're in the clear. I'd be happy to help you with this.

In his comments in a more recent thread, Christopher Slye made it pretty clear that it's OK as long as both client and designer have licenses for the font that allow the necessary number of copies:
http://www.typophile.com/node/72288/#comment-420763

Yes, obviously.

Thanks, everyone! I've found a designer, who will customise the guillemets for me.

BTW, AFAIK you can also transfer the license from the client to the designer and back again as needed (meaning only one of them would be allowed to use the font at a given instant).

hhp

Yeah, but the Transfer Forms are very hard to find… And processing them takes time.

The EULA is hard to figure out. So too are the modification rules, esp. when a third party is involved.

From:

http://www.adobe.com/aboutadobe/antipiracy/ff_faq.html

You are allowed to use font manipulation software to modify the font software to produce “derivatives” of fonts licensed from Adobe, as long as you use the derivatives in accordance with the same licensing terms that accompany the original fonts. For example, you can use Macromedia Fontographer or Pyrus FontLab to customize an Adobe Font for individual usage, but you are not permitted to distribute, sell, or give away, the derivative work, and the derivative work counts as one of the permitted number of uses.

and

A consultant may solicit their services to companies who have legitimately licensed copies of Adobe font software. The work product of the consultant must remain with the company. The consultant cannot (i) take or keep a copy of the company’s original, localized, or customized version of the font software, or (ii) distribute any original, localized or customized versions of the font software.

I *think* all they're trying to do is to make sure that the number of extant copies are all paid for; that if a new copy is made, either it replaces an existing copy, or a new licence is purchased for that copy. But who knows?

If somebody wants to know how the customised version turned out, here is a sample text (PDF). The font was customised by J. Weltin and I'm quite content with the result.