Create a bulletin for the same set of patches, e.g. '2012.04 April'
Apply it to all computers
Computers will install the updates at different times, as defined through cloned Default Software Update Plug-in policies. Cloned policies are applied to targets, and computers not targeted by a cloned policy will continue to receive the Default Software Update Plug-in policy.
For example, clone the Default Software Update Plug-in policy three times, and call them Software Update Desktops 1 a.m., Software Update Laptops 2 a.m., and Software Update Servers 3 a.m. Then apply each to Desktops, Laptops, or Servers, as appropriate. Any computers not caught by your method for defining laptops, desktops, and servers will continue to update and reboot according to the Default Software Update Plug-in policy.
Having approved the April patches as part of the 2012.04 April policy, the desktops will install and reboot at 1 a.m., the laptops at 2 a.m., your servers at 3 a.m., and all others according to the default policy's settings.
This should be sufficient to meet your needs, since each Software Update Plug-in policy can have new start dates, and you don't need to modify the bulletin policies.
Does this work for you?