Thanks a lot for the new suggestions.
I indeed forgot to mention that for the Oly FL-LM2 mod, I followed the instruction from this topic http://wetpixel.com/forums/index.php?showtopic=60039 with some modifications, given the different strobe model. Perfect job by the way!
I tried to double the fibers to trigger a single strobe, but did not work, I think indeed that most of the problem is the low amount of light available at the origin. I did my best to perfectly align the two leds with the holes in the housing, and in adjusting the angle in order to get them as straight as possible. Taking pictures in a mirror with the camera in the housing, I can clearly see the two white spots out of a full black picture (they do not produce enough light to illuminate the entire field of course).
I gave up with the DIY of fibers, so I ordered two Sea&Sea optical fibers (last model), hopefully this will help the TTL in any case.
I still want to get the LED strobe working, I am going to change the LEDs with the red ones.
Additionally, in the strobe there are two power lines one at 3V and another at 5V, can one use this second one to power a more powerful led? However I know that not only the voltage is important, but also the current and I have no idea on how many mAh I may drive to the LEDs at moment.