Here is my suggestion, incorporating the previous contributions also:

Heat up head of bolt with small and pointy but hot flame (watch out not to
melt alu), next once the heat has transfered to the threaded stuck part ,but
alu is solid, hammer the bolt head in as to nail it in to the thread.
Obviously you dont want to pulp the threads (so dont use a sledge), just
loosen it, apply WD40 or similar and keep repeating these steps.
After 2 o 3 of these cycles carefully try with a wrench ; try both
directions , yes tighten it ,and move bolt if reasonable force will do it.
Again dont force, dont twist off bolt head; be really carefull.
Start the heat procedure again. Persistence pays off, and you should get new
bolt , but this time use antiseize compound suitable to location (galvanic
reaction).
Drilling out is the last resort and sort of problematic because alu is much
softer than S/S and drill bit tends to wander. Start with small bit right in
center.If you apply penetrants through that small hole to the deep end of
bolt/hole it just may come off .
Remember you are the skip ,you are the boss not the bolt!!!

cheer miks