Override "ssh" to specify IdentityFile based on the host, but not in ~/.ssh/config. See http://serverfault.com/questions/450796/how-could-i-stop-ssh-offering-a-wrong-key/450807 for some background. Not really tested. Also, this is very naive and assumes your host does not appear in the other arguments.

override-ssh-for-identities.sh

# A wrapper for "ssh" to avoid having multiple keys sent. Leave the
# IdentityFile options out of your host sections in ~/.ssh/config. This
# function adds them based on the arguments.
function ssh {
	local new_args=();
	local arg;
	for arg; do
		if [ "$arg" = 'purplepixelhost' ]; then
			new_args+=(-i ~/.ssh/purplepixelhost.id_rsa);
		elif [ "$arg" = 'qiwib0xorz' ]; then
			new_args+=(-i ~/.ssh/matthias.id_rsa);
		fi;
		new_args+=("$arg");
	done;
	echo command ssh "${new_args[@]}";
}