I just put something a little heavy on them to stop then sometimes flipping forward.
With the Festool clamps I find the biggest PITA is when the bars fall down when doing repetitive work. A-clamps fix that problem and for the occasional time I want to keep them 90degrees its easy enough to weigh them down with a drill or hammer.
Its a neat idea IMO you have there, would be great to see them injection molded and under $5 a unit. I find when I want a traditional holdfast as your thingy turns the festool clamps into - I go for a Kreg hold-down clamp. A Kreg and a benchdog or 0.75''knob to hold it down from the bottom make for a really slick $30 auto adjusting holdfast. I really prefer it over the Festools.
Your units are nice in that its easy to move clamps around the table with little fuss - but I personally find its rare in to do many different setups for repetitive work at my MFT. When I'm doing repetitive work, I usualy have enough work to set up a Kreg/ If you can get the cost down to a reasonable level ($3-5 a clamp holder) I might buy a few. As it stands, its a neat idea, I'm sure I'll toss one or two together to see how they are.
The A-clamps work really well to keep the 'top' of the clamp above the table - if you want 90degrees, as I said above, just put your drill there when clamping up.
I may be missing something though?