For reference, I do exactly this: keeping a laptop in a safe deposit box. I visit the bank and hang out in the privacy room for a while when I need to sign a transaction. It's encrypted and has no battery. The price for the larger box isn't a lot. I think it's about $90/yr versus $50/yr for the smaller one. Either way, if you're going to the effort of putting your laptop in a safe-deposit box, I"m sure $100/year isn't going to break the deal for you.
At my bank, the privacy room has an outlet. It's because they have one of those calculators with a small paper/receipt printer in the room which requires power. I don't know if that's normal, I didn't shop around. But, I made sure beforehand that the computer works with the power cable but no battery.
I asked the bank manager if they had the option of requiring two signers at once to access the box (perhaps, to open a safe-deposit box for the company). They misunderstood thinking I wanted multiple signers. We later clarified they don't have any way to enforce having multiple people present to access the box, though they do keep strict records of exactly what time the box was accessed, and ID the person who accesses it (and the employee who helped). Not ideal, but at least it still requires collusion of one of the signers and a bank employee in order for tamper evidence to be skipped/destroyed.
Having the ability to put the wallet encryption key on a USB key is interesting. I like it for the offline computer (for the same reason I like it for the full-disk encryption). But for hot wallets, I'm not sure I like it as much. It's a lot easier to write a process to copy key files every time new media is inserted than to install a keylogger remotely (though, both are still non-negligible risks). If you do it, you'd have to make sure that removable media is thoroughly protected, so that only your user account can access it and no other system users (which is possible, but I don't think it's default, and may be inconvenient to setup and use).
Sorry for slow motion conversation here, but....
Obviously in many ways the ideal solution to your multiple signatories problem is to have multiple people hold individual safe deposit boxes, and use multisig. Now, if you want to be really secure - perhaps a really high value wallet where it's important to guarantee that none of the box holders can subvert the system by tampering with the laptop - then having a laptop in each of the boxes is justified. But in some cases it may be an acceptable solution to go for smaller boxes and a single laptop - particular if larger boxes are hard and/or expensive to obtain.
Consider the case of a high value wallet held by an individual, where the n-of-m boxes - perhaps even in different cities - are all held by the same person, but are there in part to slow down withdrawals to make it considerably harder to successfully cooerce the owner into a withdrawal. In that case perhaps USB keys in safe deposit boxes might considered adequate.
BTW, you're lucky to get a decent sized box for $100. In the UK, banks are pretty much getting out of the safe deposit business. Increasingly, banking is electronic; it's no longer about storing large reserves of cash in secure vaults, so increasingly as they move to new premise they no longer have the kinds of vaults with space to store customer boxes. At most, they need somewhere secure to store some cash to top up the ATMs - but bank vaults are a thing of the past. I pay more than double what you do for a box at a dedicated safe deposit facility - and it's not big enough for a laptop.
It's true, it depends on the shape of the boxes, too. Although mine doesn't, I believe some faciiities have small boxes that have a decent area but a very small height. I assume these are intended to allow documents to be stored without folding - but would would probably work well for storing a netbook or ultraportable laptop, too.
roy
? What a mind screw. Eventually it might be ok as I'm slowly figuring out ways of getting things done. 
