Yes you are correct the bulb capsule - glass or steel - has to be broken or breached to open and let the mercury escape, if it has then contact with the outer housing if contaminated may contain enough to be harmful especially if your lifetime cumulative count is already high. The unit in general good condition shouldn't pose any danger to most people. BTW The trunk lights are based on the same system switch capsules.
I guess you could rebuild a mercury switch unit with the guts of a bb unit but why risk releasing the contents in the process, then the State and or Federal EPA has to get involved and clean it up as a hazmat site.
If I had a mercury based under hood unit I would carefully package the original unit in a sealed container like a glad-ware sandwich box, then bag it in a zip lock, mark toxic - mercury and then store it someplace safe and out of kids reach if keeping original parts is important to you.
Then I would hang one of the decent replacements on your hood. The one Ricks sells is the correct size and shape (2x2 hanger) and uses a bb switch. Even replace the other style with the shorter hanger and the hole for heat escape with the standard 2x2. You can't see the hole when the bulb is installed and if it bothers you then drill a hole in it.
The fact your vehicle got one or the other style was luck of the draw at the assembly time, which suppliers train came in on time. No one will notice or care IMHO unless you have a trailer queen for judging and where is the fun in that?