I haven't tried it, but the following might work:
you create a document variable to hold the full text you use a DOCVARIABLE field in the Mail Merge Main document for this field instead of a MERGEFIELD field
in the MailMergeBeforeRecordMerge event, you
a. temporarily insert the MERGEFIELD field in a place that allows you to extract its result easily (e.g. at the beginning of the document)
b. if necessary, update the Field (e.g. Doc.Fields(1).Update)
c. retrieve the text (Doc.Fields(1).Result.Text
d. delete the field you inserted
e. manipulate the text as required
f. put the result in the Document Variable
g. if necessary, update the DOCVARIABLE field.
Be warned - I can't remember the details but some versions of Word don't play nicely with some characters in DOCVARIABLE fields, particularly if they are inside tables.
Otherwise, I think you would have to try to open another connection to the file (e.g. it might be possible to do this using ADO and the Jet/ACE provider+Text file IISAM), which might make it easier to extract the text, but you would have to have a mechanism (such a s a unique key) to enable you to retrieve the correct record via ADO as is currently selected in the MailMerge.