You won't get anywhere going with a different compression plug. The plug on this design is almost redundant. All it does is pull the stem against the frame. It is NOT used to manage the tightness of the bearing assembly. The lynchpin of this design (pun intended) is the screw-on ring and the pin. The degree to which you tighten the screw-on ring is what determines the tightness of the bearing assembly.
As such, I think you are stuck with the proprietary parts unless you want to switch out the stem and go with a regular set-up altogether which would look ridiculous.
Do not be tempted to overtighten the threaded ring. If you start bending the two-pin tool, you have overtightened. My thinking is that overtightening then puts too much pressure on the surface where the aluminum cup sits in the carbon top tube (well aluminum in the case of the older 675's anyway) and you will be more prone to cracking the carbon.
The Look rep here in Canada suggested putting a dab of loktite (blue not red) on the threading to help keep it in place. This did help things when I did it (it doubled the mileage before the thing got loose again) but it made it difficult to take the assembly apart again (you had to heat up with a hair-dryer).
As I said previously, still love my Look 675 but that headset design needed a few more test cycles before it reached market. I wonder whether any of the rider who took this on Paris Roubaix experienced the same kind of looseness.