Will an inline tankless heater help my 50 gallon water heater fill my 70 gallon tub?
I believe you are over estimating how much hot water is needed to fill the tub. It will be filled below the overflow drain that should be built into the tub. You can reduce the total by the size of the person getting into the tub. And most importantly you can reduce the amount of hot water by any cold water you include to regulate the temperature, perhaps you are only using 2/3 hot water to 1/3 cold water.

The important question you'll want to determine is how quickly the water heater can regenerate a full 50 gallons of hot water because you may want to heat the bath back up after a while, or use hot water for some other purpose after taking a bath. If you find yourself near the threshold of available hot water, you may find it easier and cheaper to increase the temperature of the hot water in the existing tank, rather than adding a tankless water heater.