your best bet for a lighter T-40, beyond having Chip Todd do the custom lighten-ing thing for you, is to get one of the solid colored basses. I scored a black on maple '79 last week for $270 off ebay last week (shipping brought it up to $315, still a steal for a near-mint black T-40) and it's definitely a little lighter than my natural ash on maple 1980 model. i had heard that the later model solid colored T-40's (blade pickup era) are the lightest though.
interestingly enough, there's a tonal difference between my '79 and '80: the black '79 is a bit brighter (seems more full-range, almost piano like) with less overall warmth and fullness than the natural ash '80, which has more of a P bass like thickness to the low mids and a bit less top end. I'm thinking of having a tech take a look at both basses electronics to make sure everything is working right, though it might be just a difference in the body woods (though i'm not positive the '79 isn't ash...). I'm gonna try some nickel lo-riders instead of the SS (which is what they both have right now) on the '79 and see how that works out.
God bless the T-40, such a great bass.