I really don't like the idea of a full-blown reserve system. I think that putting some kind of claim will really complicate things in the long-run and leave some kinds of bad feelings. I do think that if some model is being used for a WIP and their claim goes up, someone else who wants to use them will steal them the moment they can. Once we stop using respect as a basis for decisions, I believe that there will be little respect left. As someone who often has a ton of WIPs at once, and who takes a long time to sort characters, I I think that there will be a lot of pressure on drivers who take awhile on their WIPs to make them or lose them, in a way. I think this will ultimately end in a lot of ill-will between drivers, moreso than the current system.
I feel that as long as a character is actively working on a sheet, the model should be theirs. A picture in the sandbox shouldn't be enough, alone, to claim a model. I would agree with that. A person who's left a sheet up, untouched, for months is obviously not actively working on the character, and I don't think the model should be off-limits to others use because of the potential someone might make them in the future. However, if you can see that the driver is actively working on someone (a post edit within the past few months, even?), even if it's going slowly, I think that it's important to respect that they're intending on using them. The grey area comes after many months of inactivity. I don't think there's any grey area with a character who is currently being worked on. We should respect each other enough as drivers to know that it's not nice to use someone that someone else is using.
If we're going to implement a system, I feel like it's important that system focus more on those character sheets that are taking longer to finish, more than those who will be finished quickly. A one month or two month reserve will not fix a problem like that, because it often takes longer than that for drivers to finish a sheet, especially when you balance sheet writing with work/school/real life and posting the characters that you already have. I honestly don't think a reserve system of any kind would fix any of these problems, really. It would only give guidelines for when a model is "stealable." It actually offers nothing but a way for ill-will to be propagated. Also, do we really think that more bureaucracy will help solve anything?
However, I do think that it would be helpful for other drivers to have some sort list of models you're planning to use for WIPs. That could help some of the issues. I think that as soon as you have a model in mind, no matter how many models you have, you could submit yourself to that list. That way people have a place where they can easily see who people are using. Even Helena's suggestion that you include the model's name in your box could solve these issues. For the list, I imagine that it's a casual informatory place. Not a claim or a reserve. No time-limits. No character limits. It also doesn't necessarily stop other people from using those models. It just helps guide you to knowing where they are, so you can draw your own conclusions or send your own messages. I don't think that competition should be involved here, either. I just think it should be a place where you say "I want x for y" so people know about it.
No matter how long the sheet is taking, I don't think that a person should get cut out of using a model because of something like a reserve. Not as long as you're actively working on them. If you want to do a board wide "Sheet has not been edited in 3 months, so this model is now fine to use" rule, I could go for something like that... but I think that most of this is unnecessary. I think that the implementation of this system will just cause a lot of drama and panic and make people scramble to write sheets that are not as good just in an attempt to get the model.. and I think it will make us talk to each other less and depend on the administrators more.
I think that as long as we keep our eyes open and our mind open, this really shouldn't be an issue. If someone is working on a sheet, don't use the model. If no one is, you can use them. If someone is, but it's been ages since their sheet's ever been touched, that's moving into a grey area... but that is the only grey area I can see in the current way of doing this. By changing something or adding something to a sheet every now and then, if you're still interested in the character, could solve a lot of issues with the grey area. I don't think models are often 'stolen' when a character is currently being worked on, and from my experience, others have no patience for this kind of behavior, either.