I've used fiber cables for transferring quad signals from an electromagnetic deflection unit into photo detectors, but haven't tried illuminating a cell with them. I don't think they will transfer enough light (brightness) to view a typical cell. At the suggestion of another forum member, I tried EL wire (electro-luminescent) and it doesn't perform well either.
I doubt the wires connecting bright LED's would influence the magnets' field in any way. The current is less than 60mA for 21 LEDs on a LED tape (@12v). Even the most sensitive Ferrocell microscope slide would not detect such a low magnetic field from the power wiring.
I have been successful using ultra-bright T-3 LEDs. You can pack 60 into a 75mm ring. That would give you one 'ellipse' every 10 degrees.