OK, I think I have a working version using WRITEPROFILESTRING and READPROFILESTRING.
Thanks again Radimus, for pointing me to this.

See the two new versions of the scripts below:
sustatus.kix
code:
 
;2003-08-17, sustatus.kix version 2.0, created by Ed van Balen.
;This script checks for the status of SUS AutoUpdate on shutdown,
;writes the AU-status to a cumulative log file,
;and sets the WOL-trigger in an initialization file if the AU-status = ”5” (Install pending).

$AUState = READVALUE('HKLM\Software\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update','AUState')
OPEN(1,'\\server-name\WOL$\sustat.log',5)
WRITELINE(1,@DATE+','+@TIME+','+$AUState+','+@WKSTA+@CRLF)
CLOSE(1)
IF $AUState <> 5
            $AUState = ''
ENDIF
WRITEPROFILESTRING('\\server-name\WOL$\suswol.ini','SUSWOL',JOIN(SPLIT(@IPADDRESS0,' '),'')+@ADDRESS,$AUState)
wol.kix
code:
 
;2003-08-17, wol.kix version 2.0, created by Ed van Balen.
;This script reads the WOL-trigger ini file, starts the applicable PC's,
;and writes this action to a cumulative log file.

OPEN(1,'\\server-name\WOL$\suswol.log',5)
FOR EACH $PC IN SPLIT(READPROFILESTRING('\\server-name\WOL$\suswol.ini','SUSWOL',''),CHR(10))
	$IP = LEFT($PC,LEN($PC)-12)
	$MAC = RIGHT($PC,12)
	IF $MAC > 0
		RUN 'poweroff.exe wol -ip $IP -subnet 255.255.255.0 -mac $MAC'
		WRITELINE(1,@DATE+','+@TIME+','+$MAC+','+$IP+@CRLF)
		SLEEP 1
	ENDIF
NEXT
More background info can temporarily be found here: SUSWOL version 2 .

I had to fiddle a little with the WRITEPROFILESTRING parameters to get it working.
The IF / ENDIF sequence in the wol.kix script appeared to be necessary because the last output from SPLIT(READPROFILESTRING) is always an empty line, and PowerOff cannot handle that.

Ed