The prototype I current have (the one in all the pictures) actually does have removeable standoffs so it is possible with the current frame design but I'm not happy with how it turned out.
It requires custom-machined standoffs (extra expense) and extra-short screws (more difficult to source) but the resulting thread depth is so shallow that the motherboard screws have very little engagement.
Thus I made the decision to switch to fixed standoffs for the production design even though it doesn't technically meet the microATX specification. And then the ATX variant was done as an afterthought since it reuses most of the parts so it inherits the same fixed standoffs.
It requires custom-machined standoffs (extra expense) and extra-short screws (more difficult to source) but the resulting thread depth is so shallow that the motherboard screws have very little engagement.
Thus I made the decision to switch to fixed standoffs for the production design even though it doesn't technically meet the microATX specification. And then the ATX variant was done as an afterthought since it reuses most of the parts so it inherits the same fixed standoffs.