In Nexuiz svn, you'll now find a neat tool called "demotc.pl". It allows some sort of "cutting" of demos (actually, it works by speeding up the part you do NOT want by 100), and it can also insert capturevideo on/off marks.
Usage:
demotc.pl cut infile.dem outfile.dem time_start time_end
demotc.pl cut infile.dem outfile.dem time_start time_end --capture
demotc.pl uncut infile.dem outfile.dem (undoes cut's operation)
demotc.pl grep infile.dem "all-time fastest lap record with (.*)\n"
The latter looks for the given string in the demo, and prints whatever is inside parentheses (the .*).
A demo generated with --capture will set capturevideo 1 when the time_start comes, and capturevideo 0 at the end.