As I understand it, the Civ3 AI generally
tries to run its Sci-slider at "maximum without deficit spending", rather than "100%" -- so there is usually also some Tax-collection on most turns (which can then be boosted by Markets etc.). Conversely, the Lux-slider is used very rarely -- but apparently not never (IIRC,
@Spoonwood or
@Bartleby posted some screenies not long ago, showing some AI towns with nonzero Lux-spending).
That said, I think (you're right that) the only way to 'force' the AI to use the Lux-slider more frequently, would be to lower the rate-cap for some/ all govs. But the percentage would have to be
very low: under a 50% rate-cap, the AI would likely still split its spending evenly between the Sci- and (virtual) Tax-slider for the most part -- so only a 40% rate-cap (if that setting's even possible?) would force use of the Lux-slider on
every turn (100% - 40% Sci - 40% Tax = 20% Lux).
Of course, that would be true for the human player as well: under any Gov with a 40% rate-cap, neither of the F1 sliders could ever be set at less than 20%, so that would also leave us with very little flexibility in running
our economies.