Yes, you can have as many custom character sets as you want.
But only one set can be loaded at a time.

In your last code, the configs you have aren't doing anything.
They should look like this ... make sure the defaults are commented out in the 16F876A.inc file in your PBP folder.
Code:
@  __config  _HS_OSC & _WDT_ON & _PWRTE_ON & _BODEN_OFF & _LVP_OFF & _CPD_OFF & _WRT_OFF & _DEBUG_OFF & _CP_OFF
And how's this look for the Months ...
Code:
DateTime:
    LCDOut $fe,$80,DEC2 d," "
    GOSUB ShowMonth
    LCDOut " ",DEC2 y,"  ",DEC2 hh,":",DEC2 mm,":",DEC2 ss
RETURN

ShowMonth:
    Idx = (m - 1) * 3
    FOR X = Idx TO Idx + 2
        LOOKUP X,["JANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDEC"],Char
        LCDOUT Char
    NEXT X
RETURN