You might be able to do some client side validation to do this as JMesa uses the bassistance jQuery plugin. I have not created any real custom plugins myself though so I am not sure what the limitiations are. Also, because the worksheet only captures the changes you would not need to be able to do server side validation for this because you would not have access to the previous or next column.
http://code.google.com/p/jmesa/wiki/WorksheetAddRemoveValidateRowsV3In general I want to revisit the error handling of the worksheet. What we have seems to work ok, but it is pretty simple. I am starting a project that will be using JMesa's worksheet heavily so I hope to be able to power use this more now.
-Jeff