Your comment regarding "not feasible" concerns me a bit. You don't get to decide what is and isn't feasible. The Captain and the conditions do that. Is it feasible to get stuck in Astoria for a month, or Gold Beach?
As suggested in various other posts, I would first do enough of a shakedown cruise where it's located for an experienced captain to sign off that it's in good shape and ready for the cruise.
Then the most flexible timetable in the history of mankind. I also don't know who will be aboard or their experience. However, I ask you to consider the long term impact on their boating interest if the first trip they find the bars (not the drinking kind, but the inlets) closed and have to stay out and bounce around in 6' wind waves and 10' swells, which can probably be done safely, but in total misery. I've known people to say "never again" after such an early experience.
And do have an agreement as to Captain's pay while waiting and as to how long they're available. If you're delayed a week here and a week there and then stuck in Brookings Harbor for a week with no immediate sign of an opening, the captain may have to fly back to Seattle for commitments and fly back to the boat three weeks later. If the Captain's rate is $300 a day and your four week trip becomes 10 weeks, that could be $21,000.
We made that trip in 17 days in perfect conditions (but that was with some stopping to enjoy so could have easily been 10 days). It can be made in less but we didn't choose to travel at night. If you choose to do so have plenty of captains so fatigue doesn't take over. We were also in a bigger boat. If you plan a 14 day trip that time of year, it can end up anywhere from 14 to 60 days and most likely to be between 21 and 30 days.
Back to feasible. You must be prepared to call the rest of the trip off at any point along the way, either due to weather conditions or problems with the boat.
That said, December is considered a bad month, but everything I say above really applies to that trip anytime of the year. We had a member make it in a new to him boat not that many months ago and he had basically good conditions, although a more seaworthy boat. We were lucky but we did it in early October.