I get your point, but it should be noted that there is little evidence subliminal messages have any significant effect outside extremely specific circumstances,

I get your point, but it should be noted that there is little evidence subliminal messages have any significant effect outside extremely specific circumstances, so this hardly seems like a real threat. More broadly, the problems you are discussing would rarely be accurately addressed by open source firmware, since so few people have the required expertise or time to actually audit that. What would be far more useful would be state authorities maintaining code review boards, and asking for auditable firmware, not necessarily open source. If a state board received access to these sources and reviewed them, that would actually fix all the problems of confidence that you mention.