Wait - you said you were practicing high harmonics, and then when you go back to lower positions you have problems. How about practicing the high and low stuff separately? When you finished practicing the high stuff, wash your filthy, sticky digits. then practice the lower stuff. Problem solved.
But...Sometimes it's unavoidable - sometimes we are forced to play passages beyond the end of the fingerboard, and we don't have time to wipe the strings beforehand. Even on the cleanest bass, some rosin will get on your fingers.
You will want to practice going from high to low, and then you'll hit trouble.
However, some things can help:
1) Keep your strings a rosin-free-zone in general - clean with a suitable product regularly, and wipe down after every time you play.
2) Wipe the strings before the passage if possible (during the piece).
3) Remember, only the lightest of touches is required with the left hand to get a harmonic to sound. If you play very delicately, you shouldn't be 'rubbing' too much rosin on your hand. Hardly any at all - if you are very careful. In fact, I worry more about getting oils from my fingers on the strings up there than getting rosin on my fingers.
4) Are you using too much rosin? You could try to wean yourself off a heavy Pops dependancy. I play in a symphony orchestra daily, and I put on maybe 1 swipe of Pops every other day. So much rosin stays on the bow! Just give it a little while to warm up. I've seen others give 3/4/5 swipes every day, and the amount of rosin-smegma that ends up clung to the hairs of the bow and to the strings is ridiculous!
Hope this helps!