For anyone that may be intersted, I've come up with a "solution" to this problem. It's not without its flaws, but it does seem to be working. Here's what I've done:
In my form bean, I have a collection of ToDoItemFormView objects. Those objects are really just a collection of Strings taht represents the data that would be found in my model objects. To that class, I've added a List called errorKeys, along with appropriate getters and setters for it.
In my form bean, I've modified the validate method in this way:
Note that, when I find a validation error, rather than adding an ActionError object to the returned collection, I add the key for that error message to the individual ToDoItemFormView object's errorKeys list. So, in this way, each ToDoItemFormView object has it's very own list of errors.
Finally, in the JSP, I do this in order to display the ToDoItemFormView objects:
Note the contents of the first <tr> element. That row displays any error messages, if they exist.
So, this seems to work - I can still display multiple errors for each record and I get them out of my ApplicationResources.properties file, which means those error messages can be easily modified. The downside is that, in essence, I've rewritten the functionality of the html:errors tag using some other means. It may not be perfect, but it seems to work and, after pulling my hair out over this for 4 or 5 hours, it's good enough for me.