Lipo uninterruptible power supply?
Good question. 

Li-po batteries suffer the longer they are at a high state of charge (SOC).

Keeping them at 3.7v will for sure slow down the degrading reaction. 

This is not at all what laptops and cell phones do. They charge the batteries up pretty full and eventually they wear out. 

Q.
Is there any simple way to modify existing Lipo chargers to only charge to the "storage voltage" or would you have to design your own charger from scratch?

A.
Yes there are some easy ways. 
Lets say you have an 81v li-ion battery made up of 22 cells. It is 82v at 3.7v per cell. If you charge it with a charger that was designed for a 72v 20 cell battery then it will only ever get up to 84v which is about 3.8v/cell. So you could use a charger designed for a lower voltage battery. You will understand if this is a good idea in a minute. 

If you can open up your charger there is also likely a trimming potentiometer in there setting what the maximum voltage output is. You can trim this down to what you would like instead. 

Next, a bit of deeper understanding about charging li-ion. 
They are charged CC-CV (constant current, constant voltage) so until they get up to like 3.85 or something they are getting a constant current pumped into them. Then after that they get a decreasing (tapering) amount of current pumped into them because the voltage is no longer being increased. The current should eventually be switched off once it is at a certain point, but anyways, for you that doesnt matter. 

You will always be in the constant current portion of the charge if you stay below 3.7v. So, as long as your charge voltage will be low, the cells are balanced, and the current is controlled, you can always just leave the battery plugged in to your current source.