All is well flat out with boards at the moment.
Yes you can drill a whole lots of holes in the middle layer or if you go for 2 layers then you can router slots half way through on each side before you glue it together.
It is hard to work out how much wieght you save with this method and you get a problem with filling the holes with glue.
Having said that here's 2 more options,
1) sand/plain or cut 1mm of the bottom and deck going by the weight of 2.6kg that you said it should take off 400g bringing it down to 2.2kg.
2)There is no reason that the deck has to be flat cut some rebates 5 to 7mm deep in the low stress areas, tips, near the ends a couple of grab groves between your feet (check Wave tray), one under the handle, you get the idea.
I have a new method under vacuum at the moment I'll let you know how it works out, it is much quicker.