We've had people use it for this exact situation. It works, but there are a few things to be mindful about.
The hottest increase the incoming water is by 30-35C (86-95F) above the incoming water. This is on the highest gas / lowest water flow setting.
Say the water coming in is 10C (50F) from the tap - the HOTTAP will get it to 40-45C (104-113F), but the water will be set to the lowest setting (i.e approx 4LPM (1GPM)). So it might take a while before the bath is fully filled up
Recirculating the Water
Another idea is to circulate the water, but using a pump. Start by filling your bath up with water, then use a pump to recirculate water inside the bath (simply put the suction hose into the bath) and whatever is coming out the Hottap goes back into the bath.
You might find it's quicker to fill up the bath from the tap, then just recirculate water, and over time it'll heat up nicely. Using this method, the longer you keep recirculating water you can heat it up quite hot - as the water inside the bath will slowly get warmer over time.
However using the pump, we only recommend a max cycle time of 20min at a time. We've seen our pumps working for more than 20min at a time without trouble but again something to keep an eye on so the pump doesn't overheat.
Protecting from Wind
Assuming the Hottap will be running for a while, and if it's in an area where it's prone to winds, there is a chance that the flame could blow out. If the flame goes out the Hottap won't automatically shut the water flow, so you'll have to keep an eye on it to make sure you're actually getting warm water coming out.