alternating table row fill-color on multi-page table with repeating header

I have a table spanning multiple pages that has a repeating header. I'd like the header to always have a gray background and for the subsequent rows on each page to alternate white and gray. The rows vary in height, so the number of rows per page varies.

A plain `fill` function looking at `calc.odd(y)` won't do it because `y` will be the row number in absolute terms, not from the start of the current page. I could display the values I want as *content* via `context` but I'm not seeing how to get at the values for consumption by a `fill` function.

Anyone know a way I can do this? Thanks.