You cannot just copy the files, but you can boot off of memory sticks. Your bios has to support it n stuff though. My stick emulates a usb zip drive when it boots from it. Pain in the but to do it though, and I get it to look like a floopy, no more. Havent been able to get hd emulation off it yet, or get my kernel to boot off it yet. oh well, the quest goes on.
Yes I agree 100% you gotta use the software that comes with your "bootable" usb drive to make it bootable, but after that you can play around with the startup files that it places there by default. I believe most right now will place ME startup files, so you'd have to fiddle to get a 2K or NT Os to work.
remember that command.com isn't the only file needed to boot. even in the old DOS days, you still need IO.SYS (readonly, system, hidden) MSDOS.SYS (readonly, system, hidden) nowadays you also need NTLDR and some other crap depending on what OS yer running.