I have this same fear.  As others have mentioned, I think the best strategy is to have a sitter and a plan in place incase someone does happen to come by.  Having them take the paraphernalia away after administration is a good idea.

I would avoid the seizure story though as it may be a cause for alarm from the passer by.  When I blast off, I could pretty much pass for napping so thats the story I would go with.  Of course sometimes we can be pretty groggy when we just awaken from a nap as well

As for the potential malevolence of a passerby, I never rule out the possibly and like to have some preparedness ready in case of such an instance.  But we must drop our guard at some points in time in order to experience things like a hyperspace trip in the middle of remote nature.  Probably a safer place than most considering, but you can always do small things to make the chances of something bad happening even less. 

Don't make yourself an easy target.  If you are sufficiently away from a path, the chances that someone is going to come close to you is pretty slim.  If they do, chances are they are't in the middle of nowhere looking for victims. 

Don't make yourself look like an easy or valuable target, this goes for  pretty much anywhere and time.   Don't look like you have much to give, or that it will be given easy.  Your attitude, alertness, confidence in your surrounding are all subtle cues that would be predators pick up and and use to determine risk/reward ratios. 

Always carry at least a good knife while out in nature.  This is a great tool for many reasons, not limited to but definitely including self defence.

All in all I think its probably safer in general than a lot of activities most of us take for granted as long as you are aware of possible problems that may arise and prepare for them accordingly.