Hyperbole aside there are some simple structural principles to consider. Side to side (lateral) stiffness depends mainly on the sidewalls. The spoke bed and rim bed are not highly stressed in this loading mode. However loading in the vertical plane is resisted most by the material at the extreme top and bottom of the rim section i.e. at the bead hooks/brake tracks and the spoke bed. Putting a row of holes through the spoke bed will certainly compromise the vertical stiffness and strength to some degree. How much I couldn't say.
As far as vertical compliance is concerned, so called 'soft riding' rims actually don't flex very much and probably not significantly. In other words the soft ride is much more a function of tyre pressure and seat stay/seat post/seat flex. If a rim flexes significantly spoke tension fluctuates significantly through the load cycle. If it's too much it leads to spoke fatigue failure. Also the rim itself is setup for early fatigue failure so a flexible rim is not a good thing generally.
If you were to drill a line of holes through the spoke bed between the spokes there is a very real danger of cracks initiating at the drilled holes and going up the side walls. And just because the rim survives for some months/years does not mean it wont happen. The longer the rim is ridden the closer it comes to failure.
That said heavy rims are heavy partly because its cheaper to build them that way and partly for reliability for heavy riders, so they are somewhat stiffer/stronger than they need to be. With that and your light weight in mind I think it would be safe to drill the holes. If you do you should take care to leave ample web between the holes, don't drill near the spokes and make the holes smooth to avoid stress risers.
I googled for drillium and found only 1 reference to drilling rims. That was a fat tyre mtb rim with heaps to spare. Even so I think its obvious that the author has forgone a lot of rim life. If you look at the distribution of material around the spoke heads you can imagine that with a bit of cyclic spoke tension the web between the holes will fail. Many of the examples of drillium you will find on the web are stupidly fragile. Its a pity they dont post the failure photos.http://fat-bike.com/2011/11/tech-drilling-fat-rims/
" onclick="window.open(this.href);return false;
In summary I wouldn't do it myself because there is little to gain and potential for dangerous failure. Regular inspection should reveal incipient cracks, but then again it could happen very quickly. I'd sell them as they are and build some light weight clinchers if I were you. That way you learn some handy skills and acquire some cheap light wheels. You can build a set at ~1300g for about $300 materials cost not including tools.