Just one thing to keep in mind, Se will bind to organic Hg but not inorganic Hg. So you can't bind dental amalgam or Thymersol with it.
Also the bond is temporary; this is a non issue if you just ate some Tuna but a huge issue if your brain is still full of Hg.
The cost of Selenomethionine is higher than the equivalent DMSA\ALA to really chelate out old deposits of Metal.
More important when real chelators are used the Se can actually to the job intended at ~200mcg per day...