well i have never had a snake and i just got my BP about 2 weeks ago and i was told to use the aspen as well... it works verry well with me and the humidity, just as long as you keep some water in the water dish you should be ok i would suggest changinbg the water every other day... my humidity is around 50- 60 depending on the water in the dish....
right now the humidity in my room is about 35 %. i use aspen and i have a 45 g tank with a water bowl that is not over the UTH. i do not use light as heat and i can easily keep my humidity level between 50 and 60%. I also have most of the top of the cage covered.
aspen is good for keeping up humidity,i find around the water bowl it always gets wet and this helps the humidity but be careful because if left wet for to long aspen grows mold very quickly. hope this helps