You can do that with a little AppleScript:
tell application "System Events"
tell application process "Scapple"
tell window 1
tell sheet 1
tell radio group 1
tell (first radio button whose description is "Landscape")
if value of attribute "AXValue" is not 1 then perform action "AXPress"
end tell
end tell
end tell
end tell
end tell
end tell
Put this at the appropriate position in your macro (that is, it should run when Scapple’s Print dialog sheet is on screen).
BTW, have you seen that Scapple also offers a direct PDF export? (File > Export > PDF)
It seems the PDF export only exports the part of the canvas that is populated by objects, not the whole thing. Maybe this is better for your purposes?