totaling the value of a custom field...

No, but you could do this really easily using the query module. You’ll need to know the actual field_id number of your custom field, then try something like this (INSIDE your exp:weblog:entries loop):

{exp:query sql="SELECT SUM(field_id_999) AS mytotal 
FROM exp_weblog_data, exp_weblog_titles 
WHERE exp_weblog_data.entry_id = exp_weblog_titles.entry_id 
AND author_id = '{author_id}' AND status = '{status}';"}

The total is: {mytotal}

{/exp:query}

If you just need a static author and status, you can use it OUTSIDE your exp;weblog:entries loop, and just hard-code those values (author_id = ‘12’ AND status = ‘open’, for example).