Providing 512MB of video RAM with VirtualBox

I'm attempting to use VirtualBox to play a game on a Windows guest machine, running under an OSX Mavericks host with 512MB of video RAM. The game requires 512MB of video RAM (although it doesn't do anything particularly intensive, graphics-wise).

The slider in the configuration screen only goes up to 256MB.

I tried

VBoxManage modifyvm "Name of VM" --vram 512

but that did not work:

VBoxManage: error: Invalid VRAM size: 512 MB (must be in range [1, 256] MB)
VBoxManage: error: Details: code NS_ERROR_INVALID_ARG (0x80070057), component SessionMachine, interface IMachine, callee nsISupports
VBoxManage: error: Context: "COMSETTER(VRAMSize)(ValueUnion.u32)" at line 570 of file VBoxManageModifyVM.cpp

As I understand it, video RAM is emulated using regular host RAM, so I don't see any reason for such a limit. Is there a way to increase guest video RAM past 256MB?

This is apparently a hard limit built in to VirtualBox. Haven't tried this myself but someone on this thread reports having raised the video ram above 256MB by editing the VM configuration file and having been met with an error message and failure to start up the VM.

That same thread ventures into some speculation as to why this limit exists. It seems a reasonable question, given that modern CPUs are likely capable of handling GPU emulation for older programs that needed >256MB of video ram. Also, given that 3D support passes much of the work to the actual video card, it may well be possible to run newer programs requiring >256MB of VRAM under VirtualBox.

Some commenters on that thread thought that there was a significant amount of complexity involved in coding this change (enough to scare off those looking to hack the changes themselves) and that missing features in the emulated video card would prevent many such programs from running even given a larger amount of emulated VRAM.