Are you planning to add any kind of "persistancy support" to UNetBootIn?

No, because:

That's a distribution-level task, and would involve lots of tinkering with boot and mounting scripts in the initrd (something that can't easily be done from Windows, and differs greatly with each distribution) to provide support for those that don't provide persistence support.

Some distributions, such as Puppy and Slax, already provide this functionality in a proper fashion (ie they create the persistent storage from within the Live CD rather than requiring it to be manually made prior to booting it) which can be used with UNetbootin.

Most don't, and the majority of those that do implement it in such a way that intervention is required during the Live USB creation stage (Ubuntu and Fedora, most notably) do it in totally different ways that I can't possibly account for - Fedora expects to have a flat, empty file with a certain name, Ubuntu expects an ext2-formatted filesystem labeled as casper-rw, other distributions need their own custom formats and naming schemes, etc.

So no, at least not until there's a "standard" notion of how persistence is implemented among different distributions or otherwise I'd need to implement it seperately for every distribution under the sky in each of their convoluted ways.