The first reason I don't think following Crossfit, in general, is going to let you gain a lot of weight (lean mass) is simple calories. The body is not going to want to gain weight in a caloric deficit (duh, right), and I know you stated that you were planning on eating plenty. The big culprit is how so many Crossfitters follow a Zone or Paleo kind of eating schedule. Great for getting lean, good for performance & health, but in both plans, unless you are fudging your blocks or really, really eating a lot of meats (especially fatty ones) you are going to come up short. I think Josh Everett said it best when he mentioned that only eating 5 blocks of steak would make him cry.
The second has to do with signaling.
I have always been of the opinion that the mechanical stimulus is the most important thing for muscle gain in the non-anabolics-using lifter/athlete. This mechanical signaling doesn't just include good old TUL (Time Under Load) but also the load itself, the workload within a certain intensity zone (i.e 75-85% or whatever), frequency of stimulation, etc, etc....secondary to this for the vast majority of people is the neuro-endocrine response, and it may be Robb and Greg (and many others) will beg to differ, but we can be gentlemen about it and I can say I'll keep an open mind, eh?
In my opinion the WOD's doesn't signal well for muscle gain. It might signal better for it than Joe Blow's chest and bicep routine right out of muscle rag central, but it doesn't signal for it as well as say Joe DeFranco's Westside for Skinny Bastards, or Bill Starr's classic Big Three program right out of "The Strongest Shall Survive", or many other intelligently designed strength routines. Following an intelligent powerlifting routine often all you need to do. Olympic lifting to a lesser extent, if you really lay into the accessory work hard.
Once again, this changes dramatically when anabolics are tossed in the mix.
A corollary of this is an idea that a few friends and I have tossed around...the WOD, as lifted from Gregg Glassman's head, strongly favors those with better relative strength than not, and that typically means lighter bodies. It's been postulated due to the organism's intelligent adaptivity, an activity pursued frequently is going to have an impact on the way the body adapts to look.
This can be seen with the old sprinter vs. marathoner argument...the marathoner can't be as successful with that extra meat, while the sprinter can't be as successful without it. It's also been seen with climbers. A friend of mine went on a long climbing vacation throughout the West, and he came back looking completely different...his legs were very, very skinny, but his upper body was very jacked. I had introduced him to some of Ross Enamait's materials the previous year, and he swore he kept up on his lower body calisthenics while on vacation. Kind of weird, kind of out there, but it's something I've never been able to completely discount.
These are just my opinions and observations, of course, your mileage may vary, and probably will.