I'm unsure whether this would be loadable by rEFIt, but it's a known issue that Apple EFI updates don't always include the latest processor microcode patches from Intel. This isn't normally an issue, but when upgrading older machines (Mac Pro 1,1 and Mac Pro 2,1) it's possible to use the same processor with different stepping levels. The system will boot, but some features (VT support, mainly) are not supported due to the mismatch in processor microcode.
Other OSes and systems with BIOS rather than EFI are able to patch the microcode at boot time -- Intel has downloads for Linux, and Windows has patches. Would it be possible to write a microcode patcher into rEFIt?