Hooray!
The test arrangement for ‘getting the IR into the acrylic from below to get an FTIR-effect’ works.
In theory this is how it looks like:
The silicone enables the light to travel into the acrylic..
and in reallife.. :/
Place the LEDs with a foam/polystyrene-mount in a flat angle (~30°) on the acrylic and glue them with some transparent silicone (I got some for 7.30€).
(the yellow –thing- is from a non-permanent marker…)
A few minutes later you can wire the LEDs and turn them and your camera on:
Your hand gets lightened up by the LEDs, so place a ‘Hunt for Red October’-DVD-Case in front of them. (turn your head to find Sean Connery in the images 
)
We get beautiful blobs – even through the 5mm 7D006 projection acrylic 
All this is done by 8 LEDs. Planned are 48 each side/96 total.
Though it is much darker than a ‘real’ FTIR there’s no frame, so you don’t have to worry about water on your multitouch-table.
There’s one thing left to say: In this test arrangement finger and LEDs are on the same side of the acrylic wich is not the case in the final table.