In general, for weight weenie information, questions, and discussion, I'd recommend joining the Weight Weenies forum.
The bottle cage bolts don't need to withstand impacts. They just need to hold the cage in place against the force you exert when pulling a bottle out. They don't really bear much of a load. I am not sure what torque specs there are aside from just tightening until snug. Aluminum bolts are almost certainly sufficient. Perhaps your steerer expander bolt is another candidate for aluminum, as that component needs minimal torque and it doesn't bear a load when in operation (the load is borne by the stem clamp and its bolts).
In comparison, the stem face plate needs to withstand shocks while riding, not to mention holding the stem together at a significant torque (probably at least 5 Newton meters per bolt). That type of bolt is more a candidate for steel, although high-quality titanium bolts have been used successfully. As another point of comparison, seatpost binder bolts typically need to take at least 5 Nm of torque as well, and I've heard of titanium but not aluminum being used here also.
Probably the highest-stress location where aluminum is frequently used are the chainring bolts and spoke nipples. Chainring bolts probably take in the region of 6-8 Nm of torque. However, they are large bolts. Nonetheless, I've cracked a couple in my career, and probably other users have as well. Aluminum spoke nipples can crack as well, especially if exposed to a lot of road salt. Current generation nipples from the higher-quality manufacturers like DT Swiss and Sapim may have mostly resolved this issue.
Logic would suggest that if you do want to go this route, aluminum bottle cage bolts are safe. You may already be aware that you have to save a massive amount of weight to make a noticeable performance gain, whereas comparable aerodynamic savings are a lot more significant.