Is there an SL wide llKey2Name() workaround?

I know there are workarounds for the non-existent llName2Key.

Does anyone know of a workaround for llKey2Name() that is not limited to live, in-sim avatar presence?

You would have to use an external database, there are several out there but here is the sample code for the one that I use for name2key, not sure if it will do key2name, I have never tried that.

//Quick hack to find and display resident's keys from the w-hat name2key database
//  Keknehv Psaltery, 5/5/06

key requestid;
string resident;

default
{
	state_entry()
	{
		llListen(1,"","","");
	}
	listen( integer chan, string name, key id, string msg )
	{
		list names = llParseString2List(msg,[" "],[]);
		resident = llDumpList2String(names," ");
		requestid = llHTTPRequest("http://w-hat.com/name2key?name="+llDumpList2String(names,"+"),[HTTP_METHOD,"GET"],"");
	}
	http_response(key request_id, integer status, list metadata, string body)
	{
		if (request_id == requestid)
		{
			integer i = llSubStringIndex(body,resident);
			if ( i != -1 )
				llSay(0,llGetSubString(body,i,i+llStringLength(resident)+36));
			else
				llSay(0,"No resident named \""+resident+"\" found in the w-hat name2key database");
		} else
		llSay(0,(string)status+" error");
	}
}

llRequestAgentData will do that. The example at http://wiki.secondlife.com/wiki/LlRequestAgentData uses the object owner, but you can plug in any avatar's key.