Are business expense reimbursements considered liabilities for the company until paid to the employee who incurred them?

It depends on your choice of accounting method. 

If your company is Cash-Basis, there are no accruals for unpaid bills. Instead, the reimbursements are recorded when paid.

If, however, your company chooses Accrual-Basis, any unpaid reimbursements are considered to be Current Liabilities (as Accounts Payable).