I've actually hiked Ford Canyon (before buying a MTB)......it's a fun hike.
The "problem" for mountain bikes is the fact that, in spots, you're scambling up boulders and climbing up rocks with a couple of sheer drop offs (if I remember correctly). Like I said, it's a pretty rewarding hike. A lot of the trail near the top also takes place in the wash itself.....you get to see what the water does to the rock and why they call it White Tanks.
Just in case you're not convinced, they have a special message: HAZARDOUS SECTION Not recommended for equestrions and mountain bikers. Hikers must use caution.
You know, if they tell HIKERS to use caution.....
They also have a sign on the trail (not at the begining but when you hit this section) that says the same thing. It's actually ridable up to the sign...which is before you really start heading up the canyon.
There is no way to "clean up" the trail to riding standards without completely rerouting it. But then again, why would anyone want to reroute such a great hike?
Just my opinion.
Then again......try it and give us a ride report.....