I really like HHI's suggestion; that's very good "thinking outside the box."
And there are plenty of other strategies. Your Realtor can help you with them.
You might also want to petition your current board of directors for a temporary variance.
You may also want to contact the Association to see if you could set some flyers up in the open house unit just for those few hours on Sunday. Or better yet, join forces with some other owners that are trying to sell their houses/condos in your community and jointly purchase a larger ad in the Sunday paper as a "parade of properties tour". People may decide to visit a place that had a larger selection of properties rather than one here....one there....type thing.
Craigslist can also be a good source to draw some possible lookers. Just post that you're having an open house from whenever to whenever.
Here's the deal...You have to think wayyyyyyyyy outside the box when confronted with specific rules. Rules were meant to be extremely bent....but not broken.
Hope this helped out a little.
Your Realtor should be able to handle that problem fairly easily.
Most buyers find homes online (such as Trulia), where you can also post open house information.
The Multiple Listing Service that covers your area also probably has an open house service for buyers.
On top of that, Craigslist is probably the best way to promote an open house. You can also put signs up on street corners around the property (local regulations may apply, your Realtor should know them.)