Electronics, Lighting, & Gadgets - Overnight runtime with P7?

I will be doing my first 400k (~250 mile) ride on March 7th. I currently have a MTE P7-D
(http://www.dealextreme.com/details.dx/sku.16092)
Which serves me well for my commute. I'll run it on medium and I can do the approximately 3 hour round trip in darkness and haven't had a problem yet.
The 400k will start at 7am so no morning light is needed at first. 6:32 is sunset so I'll be using my light a bit before that until about 8 am the next day which means 13 hours runtime. Note: DST starts during the ride just to add a bit of confusion at control/checkpoints to what the time really is after I've been riding for 18+ hours.

I'll hopefully have kept up with some other folks so stopping every few hours to switch batteries won't work out. Since I can't make a multi cell holder myself I was thinking about getting one such as this.
http://www.turboferret.co.uk/Photos/battery81.JPG
turboferret (http://www.turboferret.co.uk/4holder.php)
Then I'd either load a wired cell blank or solder the external pack to the flashlight +/- through a grommet and such. Technically this should get me 12+ hours runtime on medium. Seems a little bit too close though so my thought was I'd carry a second 4pack and switch it sometime during the 13 hour night ride when we stopped at a control/checkpoint.

Am I going to totally hose my P7? I plan on bringing my fenix L2D as a helmet light but also as backup to the P7 and there are people who ride all night just on the L2D on medium or high.

It'll be fine, check out my thread in sig as its essentially what you want to do. Make sure you get a 4ps1 pack, anything else will be too much voltage. No real danger to your light short of wrecking it during modification which is unlikely unless you're really careless.

You can wire the 2 4-holders in parallel for 8 cell capacity or just use quick disconnects and change the packs easily. One thing to be careful of though, use protected cells and charge them all the way up before putting them in the pack. With it being a 4ps1 pack you shouldn't need to fill the pack up all the way either. You can run the 4 out of juice and then install just 1 or 2 on their own to get a few more hours.

Thanks it sounded good in my head but sometimes things like that don't work in practice. Hopefully keeping it on medium most times will avoid overheating issues from being on for 13+ hours.

I actually almost got a 2 18650 holder also but like you mentioned I could just use the 4x one with 2 cells when I don't want to load it up :)

One other thing having an external pack should do is allow me to have a remote switch to change modes. hmm it might be worth it just for that.