With regards the earth plug, if you were doing this in the comfort of your home, as some have suggested, how about cutting down a bolt that is fractionally too big to fit the pipe. It would only need to be a few threads long. Then, work the shortened bolt into the end of the copper pipe, which being soft will have the thread cut into the pipe. Now you have a means of sealing the end which you can simply unscrew. The charcloth can also be stored within until the time comes to use it.
Furthermore, drill a small hole through the bolt head flats (where a spanner would be placed), from one side to the other, and you can thread a piece of wire through to form a loop in order to retrieve it from the fire, and to fix to a lanyard when it cools down.
I think that this pipe idea is great, as who needs to produce vast quantities of charcloth at one time, apart from that guy selling it on e-bay!!! It is much more handy and easier than carting a nescafe tin around. I cannot wait to try this out in the local forest here in germany!!