The prussik knot is not intended for that kind of lashing. It may work itself loose if there there is no load and much movement on it. In most situations it may be possible to tighten it before the tool is lost but it would be uncomfortable to check and tighten it frequently. If the tool is in the loop itīll dangle which is IMHO a bad thing.
BTW: I really liked the idea of lashing equippment to the shoulder strap of my backpack. I was quickly cured by trying it in real life.
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If it isnīt broken, it doesnīt have enough features yet.