There's also a 2 day B.A.S.S tourney this weekend. I'd forget about Saturday on either lake, with about 100 bass rigs working L. Washington alone (since they can't enter Union thanks to Open Day) the fish are gonna see some serious pressure this week.
What you could do though (since the bass boats can't blast off until 6 or 6:30) is launch out from the Union ramp super early Sunday morning, get on top of your spot, and work that one area all morning. Those guys are gonna be all over the lake and pounding it hard. I'd find the best rocky bank you can think of and be on that spot before they even leave the launch down in Renton LOL!!!