Jason:
Not certain about Active Web Pages, but on a regular VFP form,
thisform.SetAll("Value",.T., "Checkbox")
should "check" all checkboxes on the form. You should check in the help or Hacker's Guide for the exact syntax.
The 3rd parameter "Checkbox" MUST match the name of your checkbox. We sub-classed an mvCheckbox, so we have to use that "name" instead.
Barbara Paltiel (pardon my spelling), suggested this months or a year ago, when trying to set DynamicColors for getting the "row" on a grid to show the current one as highlighted instead of just the field in the selected column.
HTH
Mike