Yeah, but Test track has 3 (I think) holding rooms....and to that effect, I think they can get away with it...most of the times I've ridden test track, they fill one or two of the holding rooms with FP guests and fill the other(s) with standby. AFAIK, the haunted mansion has only one stretchy room. I still don't see a FP working on the haunted mansion without ruining the standby line.
Now, having said that, I am a FP hater. I think it's an awful system that has ruined traditional theme park progression. We've been through it a million times, but to make FP work for you, you can't experience the theme parks in a normal circuitous fashion. You have to go back and forth from one end of the park to the other several times, in order to get any value out of it. Anyone that touts the FP system while claiming they can circuitously enjoy a theme park is smoking a recreational chemical.
Test Track has 3 holding rooms, 2 are used to combine standby and fastpass, and one is "used" for single riders. Test Tracks capacity is less than the Haunted Mansion, but I wouldn't be surprised if the demand is comparable.
Haunted Mansion could realistically work with Fastpass, but it would be unnecessary every day. To me, the break even point would be 30 minutes. If the wait time is going to be 25 minutes, and then Fastpass is turned on, the wait will presumably jump up to around 40 minutes+, validating the use of Fastpass.
Touringplans.com can project wait times at different points in the day, I have to believe Disney is capable of doing the exact same thing. If their attendance projections exceed a certain number they should be able to activate Fastpass for "seasonal" attractions at the beginning of each day. This is effectively what is done with Living with the Land and what used to be done with Star Tours.
Additionally, Fastpass does not work for shows, it should be eliminated on all shows that currently have the machines as it is not actually used for them anyways.
Here are my suggestions for Fastpass
Magic Kingdom:
Daily
Jungle Cruise
Splash Mountain
Big Thunder Mountain Railroad
Peter Pan's Flight
Winnie the Pooh
Space Mountain
Tomorrowland Speedway (New)
Seasonal
Buzz Lightyear's Space Ranger Spin
It's a Small World (New)
Pirates of the Caribbean (New)
Haunted Mansion (New)
To eliminate:
Stitch's Great Escape
Mickey's PhilharMagic
Epcot:
Daily
Test Track
Soarin'
Seasonal
Living with the Land
Mission: SPACE
Maelstrom
To eliminate:
Honey I Shrunk the Audience/Captain EO (in theory these could be converted to seasonal machines for Journey Into Imagination)
Hollywood Studios:
Daily
Star Tours (this is assuming the popularity of 2.0 is much higher than the current version)
Rock 'n' Roller Coaster
Tower of Terror
Toy Story Midway Mania
Seasonal
The Great Movie Ride (new)
To eliminate
Lights, Motors, Action Extreme Stunt Show
Indiana Jones Epic Stunt Spectacular
Voyage of the Little Mermaid
Animal Kingdom:
Daily
Expedition Everest
Kilimanjaro Safaris
Seasonal
Dinosaur (this could just as easily be Daily, but there are some days when the wait is less than 20 minutes)
Kali River Rapids
Primeval Whirl (this needs to be used far more often, as it's never used now)
To eliminate
It's Tough to be a Bug
Now for the most part, anything under Extinct has been turned off for a while now, although occasionally they'll turn on the Voyage of the Little Mermaid machines. I'm looking for Disney to remove Fastpass from 7 attractions and add it seasonally to 4 attractions, and daily to 1 attraction. I'm also looking for a few attractions to move from Daily to Seasonal (Buzz Lightyear, Dinosaur), and one attraction to move from never used to Seasonal (Primeval Whirl).