Is it safe to put a lot of makeup powder in my hair to conceal bald spots?

Using powders to conceal bald spots and thinning
should be perfectly fine.  Many people
use this method to help with small areas of thinning.  This is safe as long as you make sure to wash
hair after use as to not clog the pores. 
Also, there are newer microfibers that are plant based and are
non-toxic.

As long as you follow the instructions (use the recommended amount, wash it regularly, etc.) there is nothing wrong with using powder in your hair, and it shouldn't affect your hair loss.

Provided it is not too heavy this will not be a problem. If it is too heavy or not washed out frewuently, then yes it could cause irritation or folliculitis. But it should not cause hair loss. You may be aware already but there are many commercially available camouflaging products including keratin fibers (Toppik etc) and oowder cakes (DermMatch) and various sprays (Fullmore etc).

The use of make up to conceal balding is common and does not harm the pores or the existing hair follicles at all