Didn't know if anyone was interested but here is my solution, as a subroutine, to determine if a date is a working day or not. If your project is working on a lower granularity than days this can, hopefully, provide a good starting point.
Code:
on isWorkingDay(_date)
tell application "OmniPlan"
tell schedule of project of document of front window
-- Check that the _date is not a calendar exception (such as a holiday)
if first item of (calendar day schedules where specific date is _date) is not missing value then
return false
-- Also check and see if the day of the week is a normal working day
else if duration of item (weekday of _date as integer) of week day schedules is 0 then
return false
-- Otherwise, it is a working day
else
return true
end if
end tell
end tell
end isWorkingDay