yes you can upload alternative firmware, without really voiding the warranty.
as long as you try to flash the original firmware back when you need to send it away for RMA there isn't a problem.
And even if, usually asus doesn't make that much of a fuss about it, unless you did crazy overclocking and things like that
have you tried olegs?
http://code.google.com/p/wl500g/downloads/list
download the "WL500gpv2-1.9.2.7-d-r2381.trx" one