well it is all in the title,
i am making a shield that will power the lime (and i'd like to avoid adding a connector and a new rail to generate 3.7 and use the battery connector (have a 5v and a 3.3v stabilised already).
but at the moment the board is not booting at all is i power the pins and i don't know if it a problem with my shield or that the board just can't do it.
Both 3.3V and 5V pins at the headers are only outputs! Avoid powering LIME from the plastic headers.
Refer to the schematic, if you have both 5V applied on the headers and a battery at the same time you will cause a short-circuit around FET3 and FET2.
There is a pad called 5VEXT which can be used to power the board. You can use any GND pin. If the 5VEXT pad is not good enough you should simply unsolder the power jack and use its pads for powering (or solder at the bottom).