We can at least make a start. Using Inform vs ZIL names for Z-code opcodes we find that the ZIL name for @output_stream is “DIROUT”. Searching through the V6 Infocom ZIL source code at The Infocom Files · GitHub I can’t find any actual uses of DIROUT with three arguments at all. The closest is this, which occurs in several places in “verbs.zil” in Zork Zero:
<DIROUT ,D-TABLE-ON ,SLINE ;-80>
The semi-colon indicates a comment, but it’s interesting that that “-80” is there at all, suggesting a partially implemented or abandoned feature.
The definitions of the opcodes were found by a combination of studying Infocom’s Z-code files and disassembling Infocom’s interpreters. Some of these obscure corner cases in opcodes were implemented in interpreters but used in no (or only one) version of the released Infocom story files. It looks like the third argument to @output_stream is one of these cases. The use of a negative number for the width in pixels suggests that the first version of the three argument implementation used positive values for something else (and “specify a window” would be the obvious explanation) then when that turned out to be insufficient and “specify a width” was needed, it was just hacked in using negative numbers.