Force resolution

Is there any way to force Mac OS classic to use a resolution not displayed in the screens/display (whatever it's called in English) control panel? I have two old macs but only one native screen (a 14-inch one, I think) to go with them, so one has to be connected to a VGA screen through an adapter (part number 513-0092-A on this page at apple.com).

• I know the mac can handle at least 832x624 @ 75Hz (that's what it goes to when connected to the mac screen, I assume it's the screen's limit through),
• I know the 15-inch screen I'd prefer to use can handle up to 1024x768 @ 60Hz (my Mac mini and any Windows PC's can use that resolution on the same screen), and
• I know the adapter can handle at least 832x624 @ 75Hz (it does allow that resolution when connected to a 17-inch or the 15-inch VGA screen - after the computer has booted to the Mac screen and I switched screens as it won't show any image if I try having the 17-inch connected through booting [it's probably too modern for the poor adapter], and only allows 640x480 @ 67Hz if booted to the 15-inch VGA screen).

As stated (perhaps less clearly) in the 3rd point: When I boot to the 15-inch screen, the mac only allows me to use 640x480, and I'd like to use higher resolutions, as I know the screen, adapter, and mac are capable of. When I boot to the 17-inch one I get no image at all, but that is of less concern as I've hooked that one up to the mini (up until recently, when I got the adapter, the 15-inch was used with the mini).

At least I don't have to juggle screens to get an image when booting on the 15-in one, perhaps I should be lucky for them to be combatible to that extent?

I've tried Jon's Commands to try set the resolution through applescript, but without success. Any other ideas?

So, um... another idea that ran through my head as I typed this... is there any way of forcing the mac to ignore the screen's signals upon booting, so that it may allow any resolution? (Not having a screen connected is apparently not a solution- the mac refuses to display an image. Of course I'd have to find a working resolution through trial-and-error, but that's another matter).

Of course, I forgot specifying which mac is having the problem, it's a Performa 6200 running Mac OS 9.1. I just checked with my Performa 475 runing Mac OS 8.1, and it also displays only "640x780, 67 Hz" when having the screen plugged-in while booting. If booting without a screen however, the following choices appear:
640x480, 67 Hz
832x624, 75 Hz
1024x768, 75 Hz (the monitor can't use this though, as it's maximum is 60 Hz at this resolution)
1152x870, 75 Hz (monitor can't display this)

If the monitor, adapter, and mac, all apparently can use a higher resolution, there should be a way to force them to use it even if the mac refuses to detect it at startup, no?