I have stored my Zefal HP pump on the NDS seat stay for over ten years. Like yours I have also wrapped a velcro strap around the assemblage. Between the compression of the spring and the wrap (for redundancy) It has been my experience that that thing ain't going no where, no how. I personally can't stand to store the pump below the top tube; I like to grab the bike by the top tube to lift the bike too much.
The one thing I worry a little about is that the opening of the pump head faces the wheel and road stuff getting into the pump head. One of these days I'll make a little cover for it, or something.