This is a little something we came up with fairly early on as well, but we still use it and it seems to still serve us will. I doubt we'll ever rid ourselves of it, unless we come up with something better to take it's spot. After any natural bid of 2NT, where no suit agreement has been made, the next bid shows NT game interest, and that hand's shortest (or most worrisome suit).
Example:
1 | Pass | 1 | Pass |
2 | Pass | 2NT* | Pass |
3** | Pass | 3NT |
* Lands Game Try, ** trouble suit