Too bad those blocks are so hard to drill. When I want to install something outside
the house that requires a hole-in-the-house, I always drill a small test hole.
One that I can patch up, if it turns out to be a bad spot..
For thick walls, I use a steel rod (Music wire?) with a flatted and sharpened end, as a long drill bit.
Maybe you could use small masonry bit to probe for the cavities inside the blocks.?.
1/4 In. x 13 In. Double Flute Masonry Bit-14044 at The Home Depot
A few 1/4" holes wouldn't be too hard to patch up..