Recasting: Bad, bad, bad idea. Even in The Five Doctors, Hurndall is NOTHING like Hartnell was.
That's just because he didn't spend enough time forgetting his lines and trying to cover it up with senile giggling.
Personally, I think they should get the creepy old guy from "Captain Jack Harkness" & "End of Days" to play the 1st Doctor.
I kinda think this kind of restrospective special needs the 1st Doctor, particularly since he's so radically different from any of the other Doctors in terms of his ultra-cranky temprament and the way he often seemed like only a supporting character, particularly during the 1st couple seasons when the show seemed to mostly focus on Ian & Barbara. In some ways, the 1st Doctor is more of an idea for the character as opposed to something specifically tied to a single actor.
On the other hand, I have an incredibly difficult time picturing anyone else trying to pull off the 2nd or 3rd Doctors.
However...I've always believed that Colin Baker got totally screwed, and would love it if he were to reprise his role in the new series one last time to give him a more fitting end. Since we never saw him between the end of TOATL and McCoy's first ep (except for McCoy face-down in Baker's costume), we never really found out the ultimate fate of Baker's Doctor, or how old he got.
We don't know how old he looked in the face but he certainly seemed to still be a slender man, something you absolutely can no longer say about Colin Baker.
Here's a retrospective idea: Have the 11th Doctor contract some sort of Time Lord brain disease that makes him go through a mental regression of all his previous incarnations, so you have all 11 Doctors played by Matt Smith. (I even have this idea for a "chameleon suit" that projects attire appropriate to the mental state of the wearer. That way, we can put Matt Smith into all of the previous Doctors' costumes.)
An alternate idea: Have all of the Doctors trapped in a maze (maybe even lost in the bowels of the TARDIS). Some Doctors cross paths with each other. Some pair off with each other. Some find a comm unit that allows them to appear in voiceover (Colin Baker). Some are never located at all or are only referred to in passing.
In addition to the dead Doctors, I also think it's overly optimistic to think that you'll ever get Tom Baker to willingly do a scene with any of the other Doctors. While he's shown a recent enthusiasm for returning for audio adventures, don't let that fool you into thinking that he'll ever be willing to play along to the extent of someone like Patrick Troughton, Jon Pertwee, or Colin Baker.
Another alternate idea: In lieu of non-participating Doctors, have them represented by companions of their era: Ian, Susan, Jamie, Zoe, Jo, Sarah Jane, Romana, (no Brigadier though

Although, I think the biggest problem with a multi-Doctor story is finding any two Doctors that could ever have chemistry to compare with the fantastic bickering between Troughton & Pertwee.