While I was doing hex-editing on a scenario file uncompressed by DigiT's program, I find that we can do myriad things on the tribute effect. By changing the resource type, we can actually "tribute" kill amount, razing or everything else you can see under the Accumulate Attribute condition. (though tributing population will increase/decrease both the recent and the maximum number)
DigiT has already added the relic option under the resource type list of tribute effect, but he did not know that the Accumulate Attribute condition actually shares the same "resource list" with tribute effect. Therefore, I have told me to implement this update in the next version of AoKTS and he has promised to do so. I believe this will be pretty useful for multi-player scenarios like RPG (e.g. loop razing level-up system or conversion level-up system in a single trigger) and Bloods (e.g. buy kill/razing/conversion amount), and there is quite a high possibility for more hidden attributes.