[whpalmer4]

You can't do it directly (yet)—they added such a feature to the iPad app, but it hasn't made its way to the iPhone.

However, if you are willing to switch to a project per client, there's a way you can do it with perspectives. On your Mac, make a context mode perspective that groups by project, sorts by due, availability remaining, status due soon, duration any duration. Sync that over to your iPhone and make sure you've got the perspectives feature turned on (see the experimental section of the settings page). With that perspective, you'll see everything that is overdue or due within the next few days, and each section of the list will be headed by the project name of the following action(s).

I don't know how many clients you have, but I would try it with a project per client for a while and make sure it really is as unworkable as you think. You can easily put the projects back into the Clients project if you don't like it. Or maybe have Clients A-E, Clients F-J, etc. Mashing all of the clients into one massive project strips you of the ability to review them individually, put clients on hold, easily spot "stalled" clients, and so on, all of which I find pretty valuable with hundreds of projects.