Wow, a 64 bit InstEd. Wouldn't that be great. Well, good at least.
It would be a fairly simple task to compile a 64 bit InstEd.
The sticking point comes with the Windows Installer SDK. Microsoft do not provide 64 bit versions (at least I can't find them) of 2 dll's that InstEd uses, specifically mergemod.dll and evalcom2.dll.
One day I may knock up a 32 bit proxy process for calling these dll's, that a 64 bit InstEd would RPC to. However, hopefully Microsoft will publish 64 bit versions of those dll's soon.
In the meantime, InstEd runs happily on WoW64 and no one has yet complained of running out of address space. So wow, WoW64.