In princiuple you may be able to do this using an IF field, a section break,
and File|Page Setup to apply the correct section formatting to each section,
but I'm not in a good position to test with a real printer and you may find
that it is difficult to set it up to produce the section formatting you
expect. But roughly speaking you would do this:

Insert the following nested IF field

{ IF "{ MERGEFIELD "<breakmark>" }" = "#" "<section break>" "" }

Use ctrl-F9 to insert the braces {}
Use the name of your breakmark field instead of <breakmark>
Use Insert|Break to insert an actual section break where I have put <section
break>

Use the mailmerge toolbar to navigate to a record where the breakmark is
"#", then use File|Page Setup to set up the paper tray for each section.

Then test with a small amount of test data. I couldn't really get it right
here when testing output to a new document - the section formatting seemed
to come out the wrong way around. But perhaps you will spot a way.

Depending on exactly what you are doing you may
a. have to use a "catalog" type merge instead of a "letters" type merge,
and explicitly insert a section break at the end of your mail merge main
document
b. be able to use page breaks and different formatting for the first and
for subsequent  pages instead of section breaks.

Another approach which may work since you're using a Postscript driver is to
use a PRINT field within the IF field. The PRINT field is supposed to send a
snippet of Postscript to the printer if the version of Windows and the
driver allows it - you use

{ PRINT "the postscript" }

but then you have to work out exactly what Postscript to send, and so on. If
you go this route, you should lookup the full definition of the PRINT field
in Word Help.

Peter Jamieson