Pete:
Here's the definition, from USGA Handicap Manual:
Tournament Score
A “tournament score” is a score made in a competition organized and conducted by the committee in charge of the competition. The competition must identify a winner(s) based on a stipulated round(s), and must be played under the principles of The Rules of Golf.
Using the above definition as a guideline, the committee (preferably the Handicap Committee in consultation with the committee in charge of the competition) must determine in advance if these conditions are met and announce in advance whether the score must be identified by the letter “T” when posted.
Routine events, such as regular play days, normally are not to be designated as T-scores because they are not significant in the traditions, schedules, formats, and membership of the club. (See eligible tournament score.)
Examples of inter-club competition scores that may be posted as tournament scores when they meet the above conditions are: competitions restricted by age, member-guest competitions, team matches, qualifying rounds for city, state, and national competitions, and competitions conducted by golf associations.
Examples of intra-club competition scores that may be posted as tournament scores when they meet the above conditions are low gross-low net competitions, four-ball match or stroke-play competitions, Stableford competitions, and club championships which are stroke or match play, scratch, or with handicap.
I think there's some guidance in there somewhere that fleshes this out even more, but I can't find it.
The bolding is mine - and is the key part of this. As I recall the guidance we got was that since we only do these once a month, they aren't "routine", and thus count as T. This was the guidance we got from NCGA. So our monthly events are posted as T scores.
I guess if you do club events more often, they fall under "routine" and thus wouldn't go as T.
And that makes sense, given the purpose of T scores in the USGA system (really for automatic adjusting of handicaps when wild outlier competitive scores are posted).
I know this doesn't help your idea though.
So, the situation you wish for really doesn't exist already - not perfectly enough anyway. You would need more T scores to count for your idea to work best... and unfortunately that's not how the USGA system is currently set up.
BUT... in the end, it might have some worth to just try calculating T-score only indices anyway. It would be interesting. And I have indeed heard of clubs, or specific events, who do this.
TH