Is it horizontal or vertical scaling that's causing problems?
Horizontally, the pasteboard extends to again your entire page width — which, interestingly enough, I never thought about before. So it would seem you are quite safe scaling up to 200%.
The vertical extent is in your Preferences: Guides & Pasteboard: Minimum Vertical Offset, and has a default of 1 inch. I wonder why your “slug” trick doesn't work; if I increase it, there is still 1 inch extra above and below it. (But changing the slug back also resets the pasteboard to what it was before.)
But you can set this pasteboard size directly from within a script.
CS4:
app.activeDocument.pasteboardPreferences.minimumSpaceAboveAndBelow = “15cm”;
(see https://jongware.mit.edu/idcs4j…..rence.html)
CS5 and 5.5: uh.
It changed; see https://jongware.mit.edu/idcs5j…..rence.html. Apparently it's now “pasteboardMargins”, and now you can set the space all around at will.