I had the same thought to add a automatic supply to the large one I have. But you definitely need to have a overflow drain. I planned on setting the whole thing in a base pan with a drain. But it's all a project I have no time for. So in the mean time, 3 years so far I refill it a few times a day.
Yep, As I see it, no matter how it is done, a humidifier, is a pet...... Some take more maintenance than others, but at the end of the day, they all need a certain amount of attention.....
In the situation of humidification, where evaporation leads to a buildup of deposits, anything that increases our chances to not pay attention to the system increases our chances to have a problem. (why I am OK with manual filling versus automation)
Different types of systems need different maintenance too. The last humidifier I had, which lasted 7 years did a great job and we didn't have to replace the filters often, because it had a pump and poured water down over the "filter" elements. However, If we didn't clean the tank out regularly, the pump would plug and it was a 2 hour job tearing the thing apart (in a way that the manual didn't recommend) to take care of the issue.
We now have a wick type system, that is doing the job just fine, takes less cleaning, but the "filters" (or elements, or whatever the should be called) need to be replaced more often since the minerals end up building up in them.
Long story short, the cost of both over the course of a few years will end up being the same, but I have found the wick, where I have to change the filters more often is actually less time intensive and the "filters" are much cheaper too.
But as with many things, your mileage may vary,,,, especially depending on water condition.
Good Luck,
pen