You can catch something even with a rubber on, its all levels of risk. I also had no idea what a stripper slide was up until a few months ago. I've never had one, cover or otherwise so I couldn't tell you anything about it other than potential issues.
Fluid exchange happens more than we think and we don't even realize it. If a guy pulls out, takes off the cover and blows on your stomach etc I bet a drop or two hits the vag. I would guess if done correctly on a guy fluid would ever be exchanged. Dick pressed flat upwards, lady pays attention holding the tip so that it never slides in or she goes too far up. FLuids wouldn't be exchanged in this case and it would be no different than a lady rubbing her pussy on your leg while BBBJ or you rubbing the tip of your dick on her chest as you COB.
If I am wrong someone let me know, but if done this way i would think fluids would never be exchanged.