Fantastic question; significant overlap with my situation. I've got a couple of disorganized observations in response.
First, planning without the team violates the assumptions in the PMBOK. I don't mean to treat the PMBOK as gospel, but I've found that pointing out that a given practice is at odds with the PMBOK is enough to get the audience to listen to my next two sentences; it is vital that the next two sentences be worth listening to. The PMBOK planning process is based on planning with the best possible information and based on planning involving the whole team. Once you eliminate the team from the planning effort, the quality of your plan will suffer. If there are operational constraints that prevent involving the team in planning, management (and the PMO specifically) are responsible and accountable for compensating for that weakness. I would suggest:
Emphasis on lessons learned, and retroactive quality assessment. After you finish the plan (or any observable sub-unit of the plan), take an hour or two to measure how close the plan came to reality and ask what can I do better next time?
Frequent, Short, Modular plans. If you know that the quality of your plan is going to be constrained, then the best way to limit the damage is to shorten the plan. Transform the one large plan into 3, 5, or more shorter plans. We currently have a master plan that is long, and then we build a set of rolling six month plans based on the master plan. A catastrophic planning failure can only set us back only 6 months or so. That also gives us more lessons learned sessions, and more opportunities to adapt our planning process to reality.
Second, you can still manage to this program plan, but the importance of Risk Management rises dramatically. The program plan is set without consultation from the team, but that doesn't mean that the PM can't have a discussion with key internal stakeholders (e.g. developer team lead, or SME) and ask,
The schedule says we'll deliver X on date Y; is that realistic?
If we fail to deliver by that date, what is the most likely cause of failure?
I'll bet you $100 that we'll miss this milestone; how much will you bet me that we'll make the milestone? (One of my favorites; when I talk risk, people stop listening, but when I phrase the question this way, people listen).
If our lives depend on making milestone X by date Y, what impact will that have on quality and cost?
What can we do to make it more likely that we'll make the milestone (and you'll win our bet)?
Just as a sidenote, I prefer not to have the discussion about likelihood and impact until we've discussed the problem and both have a reasonable understanding.
If Program Dates are edicts, then schedule risk is a major component of your risk mangement. Discuss all the schedule risks, assess the likelihood, and assess the impact in terms of the number of weeks delayed. This is an ideal situation for a monte carlo analysis. This supports a strong brief to external/senior stakeholders.
The program plan shows the next milestone as Date X; we are Y% likely to meet that date. The overall program plan shows program completion on Date W; we are Z% likely to meet that date. If we are willing to spend $N, we can improve our chances by P%.