Is the key_buffer_size applicable to myisam tmp tables?
No, not at all.

Every time, I see a temp table materialize on disk, I see something like this:

-rw-rw---- 1 mysql mysql    0 May  8 17:44 #sql_22d5_2.MYD
-rw-rw---- 1 mysql mysql 1024 May  8 17:44 #sql_22d5_2.MYI


.MYI is always the default size.

On disk, they must be using the MEMORY storage engine. Again, there are no indexes created in conjunction with the temp table.

If you are concerned about many temp tables, use a RAM disk (See my post  Is it bad to create many mysql temporary tables simultaneously?)

Your goal should be avoid making temp tables, regardless of mapping tmp tables to disk or RAM.

See my other posts on this


Jan 04, 2013 : Is there a MySQL engine or trick to avoid writing so many temp tables to disk?
May 10, 2012 : Why is MySQL is creating so many temporary tables on disk?


Going back to your question: Since all your data is InnoDB, you can make key_buffer_size 8M