There's a marking—borrowed from poetry I believe—that has become relatively standard to indicate a note that should be unaccented. I've seen it in Schoenberg especially, but some other composers as well, and it looks like this:
It's used specifically for notes that would normally have some kind of metric emphasis (such as, in this example, the downbeat of a measure), when that emphasis should be avoided. The marking exists in notation programs, however I think that's because it's a standard organ-specific marking for using the heel of the foot rather than the toe. Although the "heel" version of the marking always goes on the notehead side of the note, I've only seen the "unaccent" written above the note regardless of stem direction.
However, I'm not sure this is the best marking for your specific situation, because I don't think a tongueless attack would necessarily be my choice just to take away a default sense of accentuation. To me, not using the tongue seems like a bigger deal than that, and I would be tempted to use a different notehead to represent it. Various types of tongueless attacks (and or pitchless air-noise) are sometimes indicated like this:
Or with a similar notehead but a vertical line instead of diagonal. However, I really think you could use any kind of notehead (such as a triangle or a diamond) to indicate the effect, as long as you don't use it for a different meaning elsewhere.
The only potential problem is that none of this is standard or common enough to just use without explanation. You will have to either explain your notation in the preface to the score or as a footnote to the first instance of it.