How to force text format for taxonomy description field

Hello,
I know that text formats can be ordered as wanted so the first (toppest) in the list is the default one for CKEditor.
So far, so good.
But, what if I want the default text format for taxonomy description to be different from other content types default text format? It seems the description field is not editable so I can't set it to use required text format...
Any clue about this?
The better would be to be able to set a default text format for a specific vocabulary.

Why that? Because I use Lexicon module to give definitions of selected terms, terms which are in a taxonomy vocabulary. So I have to upgrade the regular 'HTML filtered' text format to use lexicon feature (and put at top of the list, so it's the default text format for all textearea fields) and create a new 'HTML filtered with no lexicon' for taxonomy so taxonomy descriptions don't have lexicon feature avoiding a circular loop. But I don't know how to tell taxonomy description field to use 'HTML filtered with no lexicon' text format.