I also had a daughter with anencephaly; she was born and died in 2003. I am unaware of any effect of salicylic acid on one's ability to absorb folic acid, or any other connection that might cause an ntd - though that doesn't mean there isn't one. You may, though, want to get tested for MTHFR, as it does reduce your ability to absorb folic acid.
You might want to do a search in the yahoo groups, I know there are some that specifically deal with anencephaly; I used to be on one called "anencephaly blessings from above" The women on those boards keep up to date with all the research concerning neural tube defects.
Everything I know is that taking 4-5mg of folic acid a day is the best way to prevent it from happening again.
feel free to pm me if you want.