The default lookup can be done clearly and in supported way as in sdk in my othe r link. Default lookup value with GUID.
Some times while working in dev,UAT and prodution and moving from here to there
or if any one deleted the record and recreated again , means in any chance the GUID got changed , then it will not work.
But one thing here is , only one record should be there in the same name.
Even if its unsupported, still very usefull.
document.all.<FieldScemaName>_ledit.value = 'Default Lookup Value';
document.all.<FieldScemaName>_ledit.value = '';
Hope it will help many more.