Can’t speak for the ins/outs, but I thought about doing something similar a while back and, unless you plan on just using a battery pack (in which case, why put it all in one case? Just carry the battery pack around) you will need a way to charge the battery itself. Charging li-ion/lipo batteries is not immediately straightforward, but small circuits are sold to accomplish this.
Also is the issue of powering on/off. The shield doesn’t have soft on/off like mainline norns, i.e. putting shield to sleep doesn’t stop supplying power to it—that is handled by the pi. You’d have to introduce some type of switch between the battery and the pi.
I believe tom whitwell has a post in the fates thread about doing this to a fates, and you could look at that, but there will be differences between fates and shield.