I have 3T gravel wheels without drain holes and they do collect a little water that you can hear sloshing around when you shake the wheel. Getting it out is a pain if you run tubeless because you need to remove and replace the rim tape, but the amount of water that comes out is less than you think. Only some rims are susceptible to this, and I think these are because they use nipple washers that create a gap at the rim interface. I've considered drilling holes in these but just haven't got round to it. I have no fear about damaging the structural integrity of the rim with such small holes, but doing a super neat job would probably require a jig to hold the drill steady. Otherwise, if you have a steady hand, just go for it, starting with a small pilot hole and graduating to the size you need. Also, drill slowly to avoid melting the carbon and use a bit suitable for the job.
