No, I would not be comfortable advising that you take a portion of the premium expense for a personal liability umbrella policy as a home office deduction.
Such policies are designed to supplement the liability coverage built into your home and auto policies by providing you with additional insurance, not against specific "hazards" such as fire, theft, and accidental property damage, but for your personal liability for causing damage to persons or property. If you read the policy language carefully, it most likely excludes liability arising from business activities, and thus would not be a business-related expense.
