Tsahi,
We don't use Open Space either, and I also considered using it just for the purposes of adding fields for mobile access -- However, I was told that when you add the custom fields to Open Space, they're only visible to the customer when submitting a ticket, and aren't visible to analysts which doesn't help me much. Our custom field is only required when the ticket is resolved/closed, along with other out-of-the-box fields that are also required to resolve/close a ticket -- (all done via status transition macros or status dependent attributes). So in my environment, these fields need to be visible to the analysts when resolving/closing a ticket.
I honestly don't see how anyone could be using the mobile offering right now, since you can't make analysts complete the same information that the web interface requires. You would have 2 very different sets of ticket data. One that adheres to the required fields you have defined in the system, and another that is missing key information.
Tammy