This is one way to swap out the password with the one in keychain, build, then swap back.

buildwithkeychain.sh

. build.properties
if [ ${usekeychainaccess} == 'enabled' ]
	then 
	shellpassword=$(~/bin/keychain.sh ${password})
	sed "s/${password}/${shellpassword}/" build.properties > new.properties
	cp new.properties build.back
	cp new.properties build.properties
fi

ant build-all

if [ ${usekeychainaccess} == 'enabled' ]
	then
	sed "s/${shellpassword}/${password}/" build.properties > new.properties
	cp new.properties build.back
	cp new.properties build.properties
	rm new.properties
	rm build.back
fi

The shell script referenced as "keychain.sh"

security 2>&1 >/dev/null find-generic-password -ga $1
|ruby -e 'print $1 if STDIN.gets =~ /^password: "(.*)"$/'