I see. Let me restate to make sure I understand: you want a simple way to rebalance a resource's work on multiple projects by changing the %-assigned for all tasks on that project all at once, correct?
I see how OmniPlan is less than ideal here, although here's a method that minimizes the manual effort involved:
1) Create a filter (cmd-shift-f) for tasks matching All of:
* WBS numbers begin with "1."
* Assigned to Resource 1
* Show only matches in flat list
(Sample screenshot below. I advise saving this filter, and then you can easily edit it to change the WBS number for different projects, or the resource for different people.)
2) Select All (cmd-a)
3) Enter in the new assignment percentage for the desired resource for all of these tasks in the Task: Assignments inspector.
If you want to modify assignment %-ages for other projects at the same time, then press the "Edit" button in the filter toolbar, change the project #, press okay.
What you'd really like to be able to do, is to select a group and somehow say: all tasks within this group should have only 30% of resource 1's time, if resource 1 is assigned to the task. And have this not only change the existing assignments, but act the same way with new assignments. I have some ideas on how you could implement something like that with custom columns and AppleScript, but it would be better if something like it was built in, absolutely.
Thanks very much for going into detail on the actual operations you need. Like Ken said earlier, "multiple projects" means a lot of different things to a lot of different people.
