RRULE for "every 4th Thursday"

A question for you all: Community and club meetings are often scheduled 
as a particular weekday, e.g.
"the 3rd Wednesday of the month".  How can this repeat rule be specified 
in iCal?
Which vendors would then support the resulting calendar?

I get about as far as:
    RRULE:FREQ=MONTHLY;BYDAY=WE

                                  -Bryce