not with text and not by itself, but a solution is workable.
indesign's data merge can't place text files as if they were merging pictures, but it can place pictures from jpg, eps, pdf... if the text which changed was a picture, say a PDF, then this would be fine.
as to telling data merge what is variable, indesign won't parse the data as such... it'll just put the data wherever the placeholders were. if you want if/else statements, these need to be done in the excel file first prior to saving as a text file for indesign.
so yes, what the OP wants is do-able, but not "off of the shelf" - some manipulation in excel is required.
Not directly, no. You could script it without too much trouble, or you could include all of the text in each row of your spreadsheet that is fed to Data Merge.
thanks for the info, I think we found a work around.