I actually do alternating colours in my implementation, although it's handled in a slightly different way.
You'd need some way to define something like colour1, colour2, bgcolor1, bgcolour2 maxcolours=2 in an options file, and then hijack could use a simple counting property to switch the colours for each line (you would use something like <tr bgcolor=@colour@> in your filename.rep line file)

The options file could also be used to associate different filename.head etc files depending on which pages are being shown (assuming that other types of pages appear in the future... =)