I dont have my 7100t yet... but this is what i did with my Motorola v300 and I assume it will work just fine with a blackberry.
Do exactly as you described...
Input the number '46645' as the phone number (list it as Main/Mobile/etc)
and for the Name I personally put "Google SMS"
so everytime I text the GOOGLE SMS, i just compose a new SMS and when selecting name/number I scroll to Google SMS and viola !
This is a nice tool, i have noticed some delay in regards to querying my search etc... but if you have the time and dont mind the 2-3 min delay it's an AWESOME tool. i also use http://wap.switchboard.com
another good info/directory site/tool