Good catch in your update - I fed it a domain I own and got:

START
NAMECHECK: AVALIBLE
END
[download]

(moments later) Hitting the URL without any parameters - http://rscript.org/lookup.php - yields

START
ERROR: ?type= stats || rank || top10 || players || rscplayers || rskb 
+|| ge || gers || geupdate || getop || bash || qdb ||
youtubeinfo || ytinfo || urban || halo3 || reach || activity || track 
+|| trackrank || trecord || tcompare || gcalc || weather ||
wowitem || wow || clan || rsforums || spell || namecheck || loginnamec
+heck || rsnews || alog || bing || ipdns || drops || wp ||
questxp || fml || mlia || tfln || translate please.
ERROR: This script is partially by micksam7, or at least maintained ev
+ery so often. In any case: Please don't eat the fish.
NOTICE: Please don't flood this script or use for purposes against Run
+eScape Rules. Your ip is sent in the user-agent and x-forwarded-for o
+f our script.
NOTICE: It'd be really cool if you acknowledge RScript if you use thes
+e scripts <3 Really really cool <333333
END
[download]

So it's not a domain availability check; player-name would be my guess.