I was wrong - seems you have to wait 1 tick after you put in a text into a bitmap before you use bmap_process();.
Code
bmap_blit(bmapBuffer_800x600, bmapPhoto_40x40, vector(10,10,0), vector(0,0,0));  //I copy a photo into the buffer bitmap
textHalloWorld.target_map = bmapBuffer_800x600; //I copy a text into the same buffer bitmap

wait(1);

bmap_process(bmapShader, bmapBuffer_800x600, mtl_sepia); //use sepia shader onto bmapBuffer_800x600 and store result in bmapShader

Don't take the cake to the party.