That would probably work, but at a high cost. If you don't run out of hot water, but don't want to wait for it, why not put in a circulation system. You don't need to run it all the time - you could use a timer, or a momentary contact switch, or a proximity sensor to restrict the amount of time it is running. They range from say around $200-400 for parts, depending on the brand and type.
Keep in mind almost all tankless systems use a flow restrictor and if you have a big tub or a multi-head shower, the reduced flow might mean it takes longer to fill the tub or your pressure could be affected. Might not, but increased flow means bigger unit.