Can you confide in your SILs, etc.?
And tell DH that the party will be on X date, when it's really on X+1 week.
Start fake-bustling around starting two days before; moan about how the house looks kinda crummy, and it sure would be nice to have finished the guest bathroom before having all those people in the house.
Get your MIL, BILs, etc., to call up and say, "We thought we'd bring potato salad!" And then he'll start the guest bathroom. You can nag him to finish before everyone gets there.
Then when he's done, say, "Let's just do this next week. I'll call everyone. I love that the bathroom's done, though!" big smooch, big reward from you.
Have the party the weekend after? Maybe he'll be too project'd out to start a new one, and you'll have the guest bathroom done.