Hi Chris- That crag looks really fun and steep. Many of my short term goals for this fall are steep routes at the Red and New, and present some of the same challenges. So I have similar training goals.
Hangboarding is always going to help, and it is the best climbing workout you can do for the price. So, as a general suggestion I think it will pay off to get a hangboard set up at home. For climbing routes like those at Willow I think you need train three key attributes: raise your arc level on steep terrain; build power endurance, and cultivate the capacity to recover better on the rests (the best of the matching jugs on the route).
Hangboarding will raise your finger strength on a range of grip types, and in the process will raise your max stregnth as Tango notes, which will aid your general endurance by lowering your effort level on those holds. Generally this implies that you would want to use decently sized grips but add weight to get the right cycle/ If you are climbing mid 12's then you could likely start with the intermediate HB program, but adapt the grips to your goals.
You can make your holds more specific to Willow by choosing on the Rock Prodigy training center, I used a range similar to this last fall season to prep for steep jug hauls at the Red:
IMR
MRP
Middle to outer part of large adjustable edge
Sloper
Large pinch
plus, I used the pinch grip as an open hand jug, with my thumbs on the inner sides of the hold and the ends of my fingers going up and over the angled top. I hang with with heavy weight added while putting my feet on a kick plate, then I alternate hanging from one hand, as though I am chalking up. So I treat it like a big matching rest, but I have an extra 50 pounds or so hanging from my harness. I would hang with both hands for 5 seconds, remove one, chalk up, return after a 10 second rest, hang matched for 5, rest other hand for 10 seconds, repeat. 5 reps each hand. [this is a ton of "time under tension" so I would not suggest this in addition to something like the maxipull workout described below.
This is tiring, and I would only do 1 or 2 sets, or would skip it if I already felt worked by the end of the HB workout.
Beyond the standard HB workout this summer during my bridge season I am focusing on open hand endurance using a maxipull board, which is essentially 4 inch pvc pipe covered in grip tape. The protocol I am using for it is basically a strength endurance HB routine. I am doing 3 sets of 5 hangs. Each rep lasts for 45 seconds, or until failure, with a 15 second rest before the next rep. Rest 5 min between sets.
Good friends of mine (who only climb at the Red, Rifle, and Maple Canyon) are endurance crag speRCTMkicksAssts and have been really helpful in getting me started on the maxipull.
I can say that I am only into my 3rd workout of my bridge season, and I only was able to add the maxipull into the end of my performance phase in spring. However, I noticed an impact on my last day of performance when I found I could rest in the middle of a steep traverse on a hard (for me) route. I can see that I am starting to get back the sort of endurance that I used to have when I was younger. And, it is super fun being able to depump in the middle of a steep route.
As far as I can tell, the key attribute of the maxipull are that it is both deep and non-positive. This means that when you initially grab it, you are engaging your wrist to hang the front of the hold, while your finger times are slightly engaged to keep from sliding down the hold. The key thing about the protocol is that even though the first 45 sec hang is pretty easy, you barely recover during the 15 second rest, so cumulative fatigue builds an epic level pump.
I like how this shape engages the wrist flexors. These are what are used when you are resting on a physically deep matching hold on a steep route, and which allows you to relax your fingers and more fully depump the muscles that you need to use on the more challenging holds on the route. There is not a single hangboard that I have seen that duplicates these attributes in a way that prepares you to hang with a relaxed grip from the "rests" of steep routes.