If you install zsh-beta 4.3.2-dev-1+20060608-1 or later:

% zmodload -i zsh/system
% print $$/$sysparams[pid] && (print $$/$sysparams[pid])
13764/13764
13764/13765
% print $$/$sysparams[pid] && (print $$/$sysparams[pid])
13764/13764
13764/13766