1. don't do it
2. especially don't write an email to do it. it's fucking brain dead to make
a formal request in writing knowing the department chair doesn't want to
do it. s/he has a reason and you don't know the reason. this could be a turf
war unknown to you that has be on-going and you are not yet taking sides.
3. if you have the urge to do it, mention this to the chair casually over
group lunch, quoting the senior faculty suggested to you making the course a
perquisite and you are not sure - you want to ask the chair's opinion.
4. stay away from this "senior faculty" in the future. he's either not savvy
or malicious.