Hi there Bryan
A quick question - the mould you are going to make - is it going to be a 2 piece mould (split down the keel centre-line)? If so, the correct professional way to overcome your problem and how they actually mould sailing yacht keels - is before the mould is assembled in 1 piece, they polish the insides of the moulds when they are separated and then lay gellcoat up (keeping the 2 moulds separate). A layer of tissue matting goes on top of the gellcoat when it has gone green, then on top of the tissue matting there is a layer of 1 oz chopped strand matt goes over the top of the tissue matt. This is allowed to go green - not hard - in other words still soft. The edges which mate the mould are trimmed flush and the two halves of the mould are brought together. Down the centre joint there is a very narrow strip of glass matting laid down, which has also been 'wetted' out with resin. This is then forced in the narrow keel area - it is again allowed to turn green - and then once its green you commence your buildup of matting and resin - without fear of you poking through and disturbing your gellcoat/release agent.
Last bit, if you have a couple of old 'inner-tube Air valves' you can incorporate these into your mould - when you are building it - in areas next to the keel. So, when you come to release the mould from your plug - and also your hull from the mould - you can inject hot water or compressed air through them. Obviously, you remove the valve interiors and plug with a bit of wax to stop the resin/gell running through. It does leave a bit of a mark on the hull, but this can be polished out.
Aye
john