What do you need to do with the data? If you're using Designer, you should
have an "Set workflow variable" action that allows you to select a field
from an item based on the value of another field.

I know I can do THAT - but I need the field to be read-only to the user. I
suppose the workflow would just overwrite whatever they wrote anyway, wouldn't
it?

I'm confused. Can you walk us through exactly what you're trying to do?

Two lists: "Treasurers" and "Chapels"

Each chapel has a ChapelID, which is a GUID.

In the Treasurers list, I want to denote which chapel they belong to by the user
giving their ChapelID. But that list also needs the chapel's name in order to
be user-friendly. The Chapel Names, however should only be typed into the
Chapel List.

I want sharepoint to insert the Chapel Name into the Treasurers list, and I
don't want the user to interfere or be able to change the name.

Why not just make a linked field called Chapel in the Treasurers list? Then
in the linked field properties, set it to display the Chapel name.

This might eliminate the need for a GUID completely, depending on your
reasons for including that field.

Where do I get a "Linked Field"? When I go to add a column, I do see "Lookup
(information already on this site)", where I can choose to get information from
the Chapels List, and even choose a column, but I don't see any option to get it
to lookup what value in that column. How would it know what chapel name to
fetch?

I meant Lookup. You specify a list and tell it what column to pick.
Essentially, it works like a Choice field but its datasource is a different
list. The good thing here is that even if the linked item changes its name,
you don't lose the association.

> How would it know what chapel name to fetch?

It doesn't know which record to fetch. You fill it in just like any other
field. I'm not sure that a workflow is going to give you the power to
iterate through a list either, though. My suggestion was that rather than
creating an arbitrary matching value between the lists, set up your list
architecture so that it maintains referential integrity without requiring
that kind of intervention.

The lookup field is just for manually selecting data from another list. I want
to insert the data automatically.

Is it possible to have an access database somewhere where two sharepoint lists
will read from the same database?