There was already a thread earlier this year about this matter. I would also appreciate such a function, but not only for instrumental: There are other reasons not to find any text, like noise or just because there won't be a huge chance to find lyrics anymore (i.e. old songs in strange languages...).
So, I'd like if I could make a selection out of something like "N [instrumental]", "N [not found]" and "N [noise]", if MB shouldn't try to search for lyrics anymore.