The manufacturers do it, and the hole is relatively small in comparison to the structure of the tube.I wonder whether drilling holes for the Rivnuts would affect the strength of a 531 or a 531c down tube?
Yep - AND a further tip - put some tape over the poinst where you are going to drill the holes - duct tape is ideal - before you make the centre punch indentation - the tape then reduces the risk of the drill slipping on the tube (you're drilling a curved surface)Rivnuts is what you require.
Blind rivets which leave a threaded insert you can bolt a standard bottle cage to.
You will need to drill the frame, which may or may nor be simple depending on your skill level and where you want to mount the cage - clearance to get the drill square-on is a consideration.
Slow start drills make drilling a tube easier with less chance of the bit skating.
A centre punch will make a small indentation and shouldn't damage the surrounding area.
Assuming you are competent with the tools, the result will be factory quality bottle cage mounts.
Here's a link to basic Rivnut kit, there are lots available.