If you send email to the
support ninjas, they may have more suggestions; their OmniPlan-fu is stronger than my own.
I'm not aware of a way to mark a resource as available for one class of actions but not another. I think my suggested approach would be to treat each server as a group, containing a "Server X <install>" resource as well as a "Server X" resource.
You could then set up a separate schedule for each "resource". Folks would need to be careful about assigning tasks to the correct one, but that should allow your overall plan to schedule as you'd like. Does that help?
(I'm not sure if folks have asked for this type of resource scheduling before in OmniPlan, which would be another reason to mail the support ninjas - they can add you to any existing requests and/or write up a new one for you on this. Thanks!)