Will placing a strong magnet near my electric skateboard Li-Ion Battery pack damage or discharge it?
As far as I know is there no direct harm to a lipo battery if you place a magnet next to it.

I have put magnets ofthen next to electronics to see what happens and never had some problems with that.

Also in the brushless motors you are going to use there are also fairly strong magnets inside of them, so don't worry.

I'm aware that this sin't a very scientific answer, but I'm not a lipo or ESC specialist. 
I'm just an experienced electronics guy. But I'm 99.99% sure nothing bad will hapen if you do this.