In case it is of any use, take a look at the TA_meta* functions in the TADpoLe library. They are used for storage and retrieval of persistent custom meta data for a draft. It adds a meta property to a draft which you can then add any sub property to that you like.
By way of example.
If you include the TADpoLe library and run the following on a draft,
draft.TA_metaRead();
alert(draft.meta.foo);
draft.meta.foo = "bar";
alert(draft.meta.foo);
draft.meta.foo = "quz";
draft.TA_metaWrite();
On the first run it will alert undefined and then bar.
On subsequent runs it will alert quz then bar.