sort bright photos from dark photos in bridge?

Have you thought of using the time captured in metadata? 9PM is night, 3PM is day. etc.

This version writes the mean value to keywords I.E. "Mean value = 114.34"

Sorry I know nothing about Lightroom, so can't help there.

#target photoshop;  
app.bringToFront();  
  
main();  
function main(){  
var inputFolder= Folder.selectDialog ("Please select folder to process");  
if(inputFolder == null) return;  
var fileList = inputFolder.getFiles(/\.(jpg|dng|tif|psd|psb|exr|png)$/i);  
if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");     
for(var a in fileList){  
open(fileList);  
var doc = activeDocument;  
if (doc.mode != DocumentMode.LAB) doc.changeMode(ChangeMode.LAB);   
var L = activeDocument.channels["Lightness"].histogram;    
var mean =  meanHist(L).toFixed(2);  
if(fileType() != "Camera Raw"){  
setMetadata( fileList, mean);  
}else{ continue;} 
app.activeDocument.close(SaveOptions.DONOTSAVECHANGES); 
 }
};  
function setMetadata( file,mean){  
var data = "Mean value = " + mean;
if ( !ExternalObject.AdobeXMPScript ) ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');  
        var xmpf = new XMPFile( File(file).fsName, XMPConst.UNKNOWN, XMPConst.OPEN_FOR_UPDATE );  
        var xmp = xmpf.getXMP();     
        xmp.appendArrayItem(XMPConst.NS_DC, "subject", data, 0,XMPConst.PROP_IS_ARRAY);   
      if (xmpf.canPutXMP( xmp )) {  
         xmpf.putXMP( xmp );  
      }  
      xmpf.closeFile( XMPConst.CLOSE_UPDATE_SAFELY );  
};  
function meanHist(hist) {   
   var acc = 0;   
   var cnt = 0;   
   for (var i = 0; i < hist.length; i++) {   
     acc += i*hist;   
     cnt += hist;   
   }   
   return acc/cnt;   
};  
function fileType(){  
var ref = new ActionReference();  
ref.putProperty( charIDToTypeID("Prpr"), stringIDToTypeID("format"));  
ref.putEnumerated( charIDToTypeID("Dcmn"), charIDToTypeID("Ordn"), charIDToTypeID("Trgt") );   
return executeActionGet(ref).getString(stringIDToTypeID("format"));  
}; 

I don't know if this will help. This script will create a csv file of mean histogram values for a folder of documents.

(Photoshop Script)

#target photoshop;
app.bringToFront();
main();
function main(){
var inputFolder= Folder.selectDialog ("Please select folder to process");
if(inputFolder == null) return;
var fileList = inputFolder.getFiles(/\.(jpg|dng|tif|psd|crw|cr2|psb|exr|nef|dcr|dc2|erf|raf|orf|tga|mrw|mos|srf|pic|pct|pxr|pdd|pef|png|x3f|rw2)$/i);
var outFile = File(Folder.desktop +"/"+decodeURI(inputFolder.name)+".csv");
outFile.open('w');
outFile.writeln("Filename,Mean Histogram");
for(var a in fileList){
open(fileList);
var area = activeDocument.histogram; 
outFile.writeln(decodeURI(activeDocument.name)+ "," + mean(area).toFixed(0));
app.activeDocument.close(SaveOptions.DONOTSAVECHANGES);
}
outFile.close();
alert("Process complete\nCSV created...\n" + decodeURI(outFile));
}
function mean(hist) { 
   var acc = 0; 
   var cnt = 0; 
   for (var i = 0; i < hist.length; i++) { 
     acc += i*hist; 
     cnt += hist; 
   } 
   return acc/cnt; 
};