Tony.
I think you did the right thing. If you have a concern, you have a responsibility to make them known to the captain.
Although there was little risk of the aircraft falling out of the sky (as you have acknowledged), the captain did put you in an awkward situation, and showed some disrespect for you and the position of F/O.
I've got 7000hrs jet time and 4500hrs as F/O on the B733. Don't undersell yourself. You've got more than enough experience to have an informed opinion.
It's apparent to me that it wasn't the 100ft that overly concerned you. It was rather the captain's attitude to CRM and conflict resolution, and how it might have a greater consequence during a more critical situation. Fair enough I reckon.
Of course captains are ultimately resposible for the disposition of the flight and that they have a wealth of knowledge and experience. Still, I think that they should consider where the F/O is comming from. F/O's are only trying to keep the captain's paperwork to an absolute minimum!, and deserve a reasonedexplaination as to why the captain has decided to operate outside Standard Company Procedures, even though in his/her personal opinion it is safe to do so.
It's just common courtesy isn't it?