Note additional brokenness in BOM addition:
* It does add BOM even when msg is ascii, just because it was in unicode() string.
* It does not add BOM to UTF8 string if it is already encoded to str().

So highly doubt anybody actually relies on it.  And keeping such addition just increases chance that anybody starts to rely on it, so it would be good to just drop it.

For 3.3, I think it would be best to move syslog packet formatting out from emit(), because emit() already contains noticeable amount of unrelated code, so full override it is annoying.

I suggested changing BOM addition so that it only adds it to %(message)s, but such change could cause backwards incompatibility. So it does not look like good idea.

But dropping BOM even in old Python *does* look like good idea, as anybody using SysLoghandler needs to deal with BOM-less UTF8 anyway,
so dropping it will not create backwards incompatibility.