That is a very contrary view to that contained in the paper I referenced where the back test results were very good indeed. Mean return estimates are merely a forecast that the next month's returns will equal the returns of the lookback period.
So they should not be set to zero. The criticism is usually about using the very long terms returns as an estimate. In Ilya Kipnis' very attractive scheme these returns are recalculated each month based on a three to 12 month look back period.
Anyway, that is the scheme I am working on and that is the only way you will be able to incorporate momentum into the mean variance model.