I've seen on other sites and the net where people have good luck cutting lead with a course, carbide tipped skil saw, miter saw, table saw, etc. So I can always cut an ingot into filler pieces and then slurry it in.... I hope...lol. You know concrete ballast just sounds like a bad idea, esp since the top of the ballast is the lowest point in the hull making it an ideal little reservoir for any intruding water. I thank god I'm in florida and don't have any freezes to worry about or good bye keel I would think. This may sound dumb but why the "strengthening glass"? If you lay and fill around the ingots and then slurry it, that should suffice, no?
No, Laying and filling around the ingots and then slurrying over it is not a complete solution. (By slurry I assume that you are suggesting using a slightly thickened polyester resin since anything thicker than cough syrup won't flow into voids and anything thinner will crack simply from the heat of the thermosetting process and will shrink away from the encapsulation membrane failing to make a bond.)
By way of an explanation of why a minimally structural membrane is needed above the ballast, the bonding of the ballast keel to the encapsulation envelope is a key structural component in the strength of the keel and hull above. The ballast serves as the 'web' of an 'I' Beam allowing the fiberglass on either side to act and flanges. When the bond is intact, this is a very stiff structure that concentrates a lot of the stresses into a very small area where the keel turns down into the encapsulation envelope both fore and aft and side to side. Over time these stress risers take a toll on the strength of the laminate in those area weakening these critical areas in the boat. Adding to that is that boats like the Compac are often trailered with with the keel resting on the trailer and supporting the weight of the boat. The bumps in the road will result in numerous repetative small impacts to the critical laminate in the area where the keel turns down into the encapsulation envelope, further taking a toll on the strength of the laminate in this area.
While popular mythology suggests that the majority of cases where keels are lost are the result of keel bolt failure, in reality, an extremely large number of cases occur due to a failure of the laminate both fore and aft and side to side of the actual keel connections. In that regard, the current fledgling studies of keel losses show that its not just bolt on keels that are being lost.
Beyond that, in a hard grounding, the ballast keel is pushed upward against the weakened laminate and the bond between the ballast keel and encapsulation envelope is stressed and breached by the deflection of the skins of the encapsulation envelope. If the encapsulation envelop is pierced by the grounding, the boat is likely to minimally potentially be un-repairable and more extremely sink. A structural membrane, ideally coupled with transverse framing, reduces the stresses on the ballast to hull bond and on the laminate at the turn-down into the encapsulation envelope providing a much higher probability of surviving a hard grounding with a repairable boat.
If you are going through such an extreme effort to improve an otherwise intact and adequately constructed boat, then it would seem that installing a structural membrane above the ballast would be a pretty cheap bit of insurance that no matter how long you own the boat, almost no matter how you abuse the boat, and no matter where you trailer it, the repair will remain solid.