Hi Drithius, I'm sorry, but I believe that you're correct and that having vmplayer use a stretched fullscreen resolution rather than centered or auto-fit is not currently possible. This is possible in Workstation, as you've noted.
After searching for a while, I'm extremely disappointed that the only viable method is to use a 3rd party application.
As the Player is a free, they probably wanted keep certain funcationality exclusive to their commercial program.
Tried out VM Workstation 8 on a 30-day license and found there is a "stretch guest display" but only in full screen and does not retain the guest's aspect ratio. If playing old games that just distorts the image making it unappealing.
If there was at least an option to choose to keep the aspect ratio or not that'd make it worthwhile but it seems the devs decided to force it on everyone. Ideally you should be able to control wether or not to stretch the display and able to keep or ignore the aspect ratio regardless if in windowed or full screen mode similar to a video player.