Nothing spectacular would happen if 40% of the dividend was being DRIPed and suddenly everyone started asking for cash. The dividend cheques would be made out to and mailed to the shareholders instead of to the trustee of the DRIP.

BCE has a shareholder dividend reinvestment and stock purchase plan that has been around for decades.  BCE decides whether the plan's dividends and optional cash contributions are invested in new common shares issued from the treasury or in pre-owned common shares purchased on the market.  Currently, the shares are purchased on the market.