I proposed something related to this, trying to get around some of the issues you mention above. Essentially, a way to have your employer re-purpose some of your salary towards an external charity.
Fixed link: https://forum.effectivealtruism.org/posts/Whztt59kSTrnRwawu/proposed-donation-mechanism-for-people-doing-direct-work-usa
I proposed something related to this, trying to get around some of the issues you mention above. Essentially, a way to have your employer re-purpose some of your salary towards an external charity.
Fixed link: https://forum.effectivealtruism.org/posts/Whztt59kSTrnRwawu/proposed-donation-mechanism-for-people-doing-direct-work-usa