This was my problem with the entire concept.

If he is leaking, it could be because he is installing the shafts incorrectly.

If you are way off, you can't move the rockers. Guess how I know that?

I'd be a bit concerned if any of the slot were in the space between the two carrier holes. The rocker forces normally are transferred to the carrier via the full width of the rocker shaft. The slot reduces this substantially.

With that said, I once had a shaft get loose. I discovered this when it had moved far enough so the slot was where it shouldn't be, but it hadn't been there long and pushing it back into place and tightening the pinch bolt got me back on the track with no damage.

Here is rocker shaft dimension

http://forums.pelicanparts.com/911-engine-rebuilding-forum/343519-rocker-shaft-pinch-bolts.html

Another idea we use in aviation; Caplugs.

Get a really heavy duty plastic plug from Caplugs that fits the bore but has a small flange that keeps the plug from going in deep.

Then, just add some non-hardening sealant like Hylomar. It would be a goopy mess when you went to remove them but it may hold in the oil.

The beauty of Hylomar is it easily wipes away with thinners, solvents, or like we use at my work, acetone.