Qemu has the -cpu switch which can be used to set CPU features, that may sometimes help. Run qemu with "-cpu help" to see what's available.
I actually used it some time ago to track down a bug that only manifested with SSE4.1.
TCG plugins are yet another possibility, take a look at contrib/plugins/howvec.c.
Although with qemu (unless you're using user mode) you'll be switching the CPU features for the whole guest OS and it's a lengthy manual process anyway.
I think most other emus only provide a CPU type in the config rather than the individual feature flags, but that may helpful too.
SSE2 was introduced with P4 though, probably not too many emus available for that.
EDIT: This thread has some more discussion (and solutions):
https://superuser.com/questions/726395/how-to … or-avx-on-linux
EDIT2: The given solution elfx86exts seems quite close to what you were asking. Won't uncover any hidden code though.
Btw it's Rust-based, so running "cargo build" will fetch ~360MB worth of dependencies.