Item Stat bonuses are already signed in regular FE8 (or at least FE8U), nothing to do with the skill system (the skill system may or may not be responsible for making them display properly, however).
For example, here's the loading asm for the power bonus:
ldrb r0, [r0, #1]
lsl r0, #0x18
asr r0, #0x18
After loading the bonus value, it's sign-extended, effectively making it signed.
Here's a list of reference function addresses (FE8U):
(They all do the same sign extension trick)
Also you'll note that there's no such thing as mov & con stat bonuses from items (or at least they do no work) (again, in the vanilla game).