[XeTeX] Coordinating fonts in text and math mode: question from a new user

[Beau Madison Mount]

Dear members of the list,

I apologize in advance if this is a frequently posed question, but I  
have found almost nothing about it -- or at least nothing at a level  
useful for beginners. If there is some standard how-to guide, I'd be  
very grateful if someone could point me to it.

I am very new to TeX and its offspring. I downloaded MacTeX for OS X  
about a week ago, and I've read a few online guides, become reasonably  
familiar with the basic commands, and experimented a bit with XeTeX  
and the babeltext and fontspec packages. I'm planning to port my  
dissertation over and finish it in TeX, and I should like to use  
either Adobe Caslon or Hoefler Text as the primary font -- I haven't  
quite decided which. I also have occasional words and passages in  
ancient Greek in the text, for which I'd like to use a Porson font  
(probably the one produced by the Greek Font Society). So Xe(La)TeX,  
with its support for OpenType, seems the obvious option. This part  
appears relatively easy to do.

The problem, however, is that I also have occasional things to be  
entered in math mode with variables and operators in Greek and in  
serif roman upright and italic. I want to use the same fonts for these  
letters in math mode that I use in text mode: anything else looks  
atrocious. I also want to use the old-style figures from the main font  
in math mode. As for the non-alphabetic mathematical symbols, I am  
quite happy to keep the ones from Computer Modern. [*] (As a bonus, I  
should like, if possible, to be able to use the true lowercase omicron  
and the "missing" uppercase Greek capitals in math mode.)

I believe that this will probably require creating a virtual font. If  
I am wrong about that and there's an easier way, I should very much  
like to know before I start the project. If am right, I'd like to find  
something like an math mode virtual font FAQ to guide me. But I  
haven't seen anything like that. Is there such a thing? If not, can  
someone tell me if not what to do, at least how best to go about  
learning what to do? What should I read to learn how to do this well?

As I said, I apologize if there's an obvious solution that I should  
know about. I can't imagine that I'm the first person with this  
combination of desiderata -- there must be many philosophers who need  
Greek and Classicists who need the occasional logical or mathematical  
symbol. Beforehand I had just assumed that this was the sort of thing  
XeTeX/XeLaTeX would do easily; but now that it comes time to do it, I  
can only find bits and pieces of information scattered in very  
advanced discussions, and no general guidance for beginners.

So -- please help!

Best,

B. Madison Mount

[*] (Although, if it should turn out to be feasible to use an inverted  
A and reversed E from the main font as universal and existential  
quantifiers, and a turned iota from the Greek font as a definite  
description operator, that would be wonderful. But if it's immensely  
difficult to do that, I am willing to compromise.)