Is it possible to edit the planetary bonus tiles in Twilight?

The short answer, most probably not.
The long one goes like this;

- Specially made or new buildings with multiplying effects to some ability DO in fact represent an indirect Bonus tile.

- It may be a good idea to make these a Super-Project or with a Placement limit of 1.

- I have yet to decide on a Precursor type of extra booster at 700%, though... considering luck is one thing. And gameplay balance another.

But if you meant having entirely new ones with different impacts, i doubt it unless you could patch the code (which i wouldn't really recommend either).

Besides, what's more precious?
Morale or Influence gained by randomly occuring percentages or spiffed up by weird tricks or even somehow, by strategic decisions?

Zyx, I don't think you answered the specific question.

Sounds more like auboy wants to edit the actual bonus tiles on any given planet, not muck about with buildings that have bonuses.

The answer is probably yes. But, It would have to be done with a savegame file with a hex editor.
It can be done with Dread Lords pretty easily. I use Hexplorer to edit bonus tiles.

There are a few threads that deal with it, some of which I have posted screen shots and given detailed instructions. Not really that difficult. If you want, you could create a class 72 planet with 72 700% research or manufacturing bonus tiles. Not really economically feasable, but it is possible.

The file structure has changed since DL, so I would not be able to give exact instructions for the expansions.

Alrighty... but, i "understood" the shorty comment as a way to formally mod such things in the usual manner;

- I certainly agree that good hexing edits would be a solution, not entirely elegant or even standard, nevertheless it does the job. If someone knwos exactly what & how to perform such tricky corrections.

- I guess the point with Bonus tiles is that they are a fundamental feature created by designers and that their inherent mission is to balance gameplay (somehow). When if somebody could alter these sooooo much, it would become a silly try. Well, anyone has a right to adjust THEIR own games and thus, should probably be able to fix whatever they wish - but, at the risk of breaking hell loose on all cylinders if not screwing up the code so bad it'd crash.

- Although, some properties or exact values still are a mystery (to me, at least). We can always ask for precision or detailed explanations (that's how **I** got to grasp many things, btw) and even, invent our own.

The magic in Modding, i believe, is in the process and all these variable discoveries we make.

Right now, i'm onto the challenge of planet surfaces & tiles re-definitions (height gaps, new queries, coloring, etc.) and having such a blast.
Just the Abyss promises to be a heck-of-ride to drop as a "Marine" terraforming option, calculated to be extremely rare and actually working as it should!

Now, THAT's a Bonus to any modders (or eventually, players!).

Forgot to ask... You wouldn't happen to have the saved-file structure & details for any of us to look at, would you?
And if so, extrapolate on DA versions, etc.