I just stumbled on this simple method in HAIs Knowledge Base while searching for something else:
http://kb.homeauto.com/default.asp?id=801&Lang=1&SID={DBC64BC7-6FE5-47E4-B040-7A3634B3ACB5}
Looks like you only need two lines of code and one link if you don't want to reconfigure to HLC (method 2 below):
<ETA: Looks like this is also the advice in the 3.2 Owner's Manual on page 29 - if I would have read that first.

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As stated in the 2.6 Owner’s Firmware Release notes (and later owner manuals), the All On/Off features in an HAI controller are not supported when the House Codes are defined as UPB.
If the All On/Off features are required, you can do one of the following:
1.) Define the House Codes being used, as HAI Lighting. This option would require you to either use HLC lighting products, or configure 3rd party UPB devices to emulate HLC products.
2.) If UPB is the “desired” lighting control method, you could take the following steps to provide All On/Off functionality:
Connect a UPB PIM to a PC (NOTE: If using the HAI PIM, you can use the 36A05-2 for this procedure).
Open the Upstart software.
If you have an existing network file, open it. If not, bullet “Create a new network file that matches an already installed network”.
Perform a network discovery to display all lighting previously setup.
Open the properties of each switch and add an unused link to the Receive Components (NOTE: To keep from possibly interfering with HAI controller functions, any link 226-240 should be used.).
Set the properties for the new link to 100% (or desired level) for the Lighting Level, and set the Fade Rate to “Snap!”.
After adding the link and properties, click on the Program Switch button to add the link to each switch.
Once complete, close the software and reconnect the PIM to your HAI controller (NOTE: Remember to reset the PIM to Message Mode.).
In the HAI controller, create two lines of programming, similar to:
WHEN ALL ON: LINK X ON
WHEN ALL OFF: LINK X OFF