Further to last night's RTFM:
Solaris 10's 'xargs' does *not* appear to support -P: xargs: Usage: xargs: [-t] [-p] [-e[eofstr]] [-E eofstr] [-I replstr] [-i[replstr]] [-L #] [-l[#]] [-n # [-x]] [-s size] [cmd [args ...]]
On the bright side (crossing my fingers), we won't be supporting the Solaris 10 boxes much longer. Well, maybe only one of them, but it shouldn't need much attention. After this summer we'll need to support just one app/DB pair of zones.