My father does a lot of turning and carving. He uses paint to seal the ends of logs and other pieces of wood to allow them to dry more slowly (checking and cracking are a result of wood drying faster in some places than others - the dry areas contract, and the fibers pull apart). He also told me there are certain specialized sealants, etc. that are specifically made for drying timber like that, but a can of latex based paint will do fine. Red Oaks (the group, not the specific species) tend to check and split more severely during drying than most others because of the way the wood is made (e.g. the way the xylem is converted into wood).
I haven't tried any burn bowls, myself, so I can't really give any advice.
As an arborist, I can tell you that taking burls off of trees creates a wound that never heals - it just grows over. Compartmentalization within the tree through chemical secretions that fill the dead inner cells is usually what keeps a tree from rotting over a wound. It basically creates a chemical "wall" that stops decay at that point. Some trees are better and compartmentalizing than others. Hard maples are usually pretty good at it - softer "hardwoods" are usually bad. Burls on a tree are usually not bad for the tree in and of themselves - they're basically an over-pumped dormant bud where the tissue just went kinda nuts. Usually it's triggered by a fungal attack or some kind of insect infestation. Sometimes they just happen for little apparent reason.
Anyway - cutting off a burl leads to an open wound - so harvesting from live trees might be something you want to re-consider. Maybe consider just taking and using the tree in its entirety. It doesn't matter what time of year you cut it off (with the exception of oaks and elms, but that's because of Oak Wilt and Dutch Elm Disease more than anything). Additionally, a tree cannot "bleed to death" - they may lose vigor, and it might look extreme (especially on maples), but only in cases where a tree is already weakened (such as in a drought, or dealing with a fungal/insect infestation) will a tree lose so much sap that it will die. Most of the time what kills a tree is not the amount of sap it loses, but the amount of living tissue around the trunk it loses (which is why it's bad to peel bark off of trees). I could on, but I think I've sounded enough like a botany text book for today.