I can assure you that drying out the container is not an issue. When it comes to moisture, how is a heat lamp any different from an undertank heater? They certainly need to have supplemental heat, so there really is no choice.
It is true that the smaller the snake, the higher surface area to volume ratio they have, and the more likely they are to lose moisture. However, hognoses inhabit almost exclusively areas with sandy soils. Sandy soils drain water away very easily. Their natural habitats are inherently going to be on the dry side. On top of that, I don't recall having any shedding issues with captive hognoses, nor do I recall any problems being reported on this forum. So basically, moisture is not really an issue!
If you are truly concerned about moisture, it is better to provide a container with a small opening and saturated paper towels, for instance. If the snake enters into ecdysis and finds that it needs more moisture to help it shed, it will certainly have the option to go into the container as it needs. As far as sanitation goes, it is far better to limit moisture, where bacteria are going to inherently flourish, to a small container that is easily disinfected rather than an entire container. It would be great if you try this method and reported back on this forum if the hognose actually uses the container. Personally, I have always try to provide a large enough water dish for the snake, regardless of species, to soak its entire body.
There are far too many variables to tell you a specific wattage. It depends upon the tank size, the distance from the lid to the floor, ambient air temperature and substrate. Personally I have always used 60 W bulbs for 10 and 20 gallon long tanks. During the winter you may need to bump that up to a 75 watt bulb depending upon your substrate. You can use higher wattages with Aspen because it does not transfer heat very easily. Sand substrate, which I really don't recommend, transfers heat easily and can potentially burn the snake.
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Virginia Herping
http://groups.yahoo.com/group/VaHS
Virginia Herpetological Society
http://fwie.fw.vt.edu/VHS
If there is a just God, how humanity would writhe in its attempt to justify its treatment of animals. - Isaac Asimov