Just an idea that I've been aiming to try - how about just adding some self-tanning lotion to the sunscreen? As long as the factor on the sunscreen is high enough it should be able to handle some dilution.
When I bought a nice self-tanner last summer (Clarins) the cosmetician actually advised me to dilute it down with moisturizer to lessen the darkening effect. This way is could be used year-round, if desired, without an abnormally dark tanning result. It did so and it worked well, and stretched out the supply of tanner considerably vs. using it straight.
Of course, the moisturizer is inert compared to the sunscreen, but maybe the actives in the sunscreen won't care about the tanning ingredients. I think it is worth a try and plan to do so in a month or so - it's still too early for suntans here in Canada.