All I can think of is to set it for every 9th day, then adjust it to the next Monday whenever it falls on Saturday or Sunday. How often would that happen? ... Well, if starting on a Monday, it would be once a month, because the pattern would be M W F Sunday -> M W F Sunday etc. or T T Sat. M.. hey, a pattern! Very quickly, it always falls on a M W F. So what do you do about M holidays, or other holidays that fall on any other weekday?
Without that problem, you could set three events to on M W F every 5th week. That would work, but would fall on holidays.
You've gotten the creative ideas. Apparently iCal can't do it with its standard settings. Until that changes, it's going to require more effort on your part. Sorry.
Maybe you need to use something else. Perhaps a spreadsheet, that you open every day to see which task is scheduled for that day. I don't know of a quick way to import a calendar, but once you've set up a template, if you do it intelligently, you should be able to customize it for any given year. After all the dates are entered, you'll have to delete Saturdays and Sundays, then any holidays you don't want. Now you can list the 7 tasks, and copy/paste them for the whole year. When closing the file each day, you can delete that day, so the next day will always be at the top. Save the Master!
Use formulas to repeat months, and copy/paste, to save some typing.