Hey,
Without knowing the complete details of how your project will be deployed, you might be able to use the following. It is written in VB Script: (Note: to use this code the form will need to be fully trusted - also, this code is off the top of my head and has not been tested.)
Sub PopRowColValue(sFile, sToPop)
'sFile - is the path to your excel file
'sToPop - is the InfoPath field value to populate to Excel
Dim objExcel
Dim objActiveWorkbook
Dim iAllRows
Dim iLastRow
Dim iRow
Dim iCol
Set objExcel = CreateObject("Excel.Application")
Set objActiveWorkbook = objExcel.Workbooks.Open(sFile)
'grab all the rows of the appropriate sheet
iAllRows = objActiveWorkbook.WorkSheets("Sheet1").Rows.Count
'set to column A
iCol = 1
For iRow = 0 to iAllRows
If objActiveWorkbook.WorkSheets("Sheet1").Cells(iRow, iCol).Value = "" Then
'tag it because it is the last row for the column
iLastRow = iRow + 1
Exit For
End If
Next
'write it to excel
objActiveWorkbook.WorkSheets("Sheet1").Cells(iLastRow, iCol).Value = sToPop
End Sub