I wonder if there is a way to safely operate a UV germicidal bulb such as a germicidal PL lamp or a clear linear low pressure mercury vapor lamp that fits into fluorescent fixtures?

You can stick it inside a glass jar as that will block out most of the UV-C.

Virtually all household materials block UVC. Something as simple as running it with the diffuser on the fixture will prevent UV exposure, especially if the diffuser is plastic of some sort. However, this will deteriorate most plastics or other organics rapidly, and they will begin to crumble in a matter of weeks or months.

If you're attempting to irradiate something, any barrier between it and you will suffice.

It is not the substance, but the dose, that makes the poison. Keep your exposure low - a few seconds at close range. Inverse square law applies, so having it 3 meters from you instead of 0.3 meters means a reduction of not 90%, but 99%

Depend what do you mean by "operate".

If just a showcase to show it running, then a box with a glass "window" is good enough.

If you really want to disinfect a room, then best is to use some kind of timer control (the timer is thebest, because it maintains the dosage consistent) or at least extension cord passing just through cracked-open door, so mainly your eyes dont get any exposure. Then of course immediately after use you should ventilate the room by open window (to get rid of the ozone; during the disinfection the ozone is better to keep in the room so it will disinfect even surfaces not directly exposed).
And make the control so it can never get activated accidentally (so disconnect from the socket immediately after use).