Editing a single field on a rendered node On a given content type called Tracking (which is also a Group), I have a title, OG fields, and a repeating text field called Log. I'd like a member of the OG to be able to enter a value and submit without entering the edit field. 

The use case is a mobile site with observers in the field. They won't be editing the entire node. They're just taking notes into a repeating text field?

Is this possible or should I seek a different solution? Or should I just rely on Comments for such a function?
Use whatever works for you. I've used the below modules to achieve this type of functionality before.

Look at Field Permissions:


  The Field Permissions module allows site administrators to set
  field-level permissions to edit, view and create fields on any entity.


And look at Editable Fields to easily edit a field when not on the edit page:


  This module allows CCK fields to be edited on a node's display (e.g.
  at node/123), not just on the node edit pages (e.g. node/123/edit). It
  also works within views etc. Anywhere a 'formatter' can be selected,
  you can select editable (or click to edit).


Or maybe Editable Views, modules:


  Editable Views lets you create Views where the fields can be edited.
  Fields show both the current value and a form to edit it. A 'Save'
  button allows saving of all the entities shown in the View.
  
  Editable fields can be added on the base entity as well as on entities
  brought into the View by a relationship. Other, non-editable fields
  may be added to the View, in which case they function normally.