Would you consider adding a custom field to tasks through the API? I'm envisioning a method whereby a developer could extend, for instance, a mobile applications's functionality by storing data that's not currently available in Toodledo such as location, etc. If a developer adds fields in a third-party app today, there's no way to store that info in Toodledo without commandeering one of the existing fields (tags, for instance).
If you allowed a custom field in the task database, then a developer could store whatever additional info is necessary in a text format (JSON). You could limit the field length if storage is a concern.
This is on our to-do list. In the meantime, what we have been recommending to developers is that they create a Notebook entry called "AppName MetaData" and store all your extra information in there as XML or JSON or whatever. Then you can just grab that notebook entry and parse out your custom data. This obviously isn't ideal, but it is better than commandeering an exiting field that the customer might be using.