You don't need to use the same IDT as in long mode. Just have a 32-bit one that decides whether to handle an interrupt (e.g. vm86 traps/emulation) or re-enable

You don't need to use the same IDT as in long mode. Just have a 32-bit one that decides whether to handle an interrupt (e.g. vm86 traps/emulation) or re-enable long mode and forward it to the 64-bit handler. But you don't have to imagine -- early amd64 Linux could do it, and there was a patch floating around that kept that feature all the way to 2.6.2x series. These days vm86 is borked even in 32-bit kernels.