Here are some general ideas, preliminary because I have not taken a close look at the code yet. For reference, here is a completed status response on a full-import from 3.5.0:
<?xml version="1.0" encoding="UTF-8"?>
<str name="Total Requests made to DataSource">1</str>
<str name="Total Rows Fetched">11287894</str>
<str name="Total Documents Skipped">0</str>
<str name="Full Dump Started">2012-04-03 17:38:01</str>
<str name="">Indexing completed. Added/Updated: 11287894 documents. Deleted 0 documents.</str>
<str name="Committed">2012-04-03 20:16:32</str>
<str name="Total Documents Processed">11287894</str>
<str name="Time taken ">2:38:31.314</str>
<str name="WARNING">This response format is experimental. It is likely to change in the future.</str>
I was thinking it might be a good idea to have two response sections in addition to the echoParams section already mentioned - one for a human readable response and one for a relatively terse machine readable response. The human readable version would be fairly open to change, and could include extra verbiage so it's very understandable for a person.
The machine readable version would have more elements, each of which is very simple, probably just a numeric value or a true/false indicator. A design decision needs to be made early - do we include all elements in every response (with the value set to zero, blank, or false), even if they don't apply to the current status? My first instinct is to include all elements, but maybe that's wrong.