What happens when you slow charge a li-po?
With lithium chemistry, if you allow the voltage to exceed 4.2v, or stay at 4.2v for any length of time, you harm the cell. 

This is why you have to have a limited voltage to finish charging. You must also stop charging once fully charged, to allow the voltage to drop back from 4.2v, and only give infrequent top-ups.

Even C/50 may exceed 4.2v on a fully charged cell.

Floating at a voltage &lt;&lt; 4.2v, for instance the 3.5v you mention, should be OK for lifetime, but the usable capacity will be a long way short of 100%, so making poor use of the expensive chemistry you have just bought. However, if you have the cell, and don't need the runtime, then a low voltage float should work.