Apparently the last row always uses <table:last-row>
from styles.xml. If this tag is deleted, it is automatically created again when the document is saved.
The only workaround I can think of is to use one style for tables with an odd number of rows, and a different style for tables with an even number of rows. You can always modify the background for a particular row by going into Table Properties.
Of course, it’s possible to embed a Calc document, as suggested at Using conditional formatting in OO writer (View topic) • Apache OpenOffice Community Forum. Calc can highlight alternate rows with Conditional Formatting.
Improvements for table styles have been discussed, so hopefully there will be a better solution in the future.