HI Anand.

In theory a great idea... BUT :- LCD and RTC has closer power supply tolerances than the PIC... ie +/-3.3v or 5v. So not as forgiving !

A decent lithium PP3 is also rated at around 900mAH so....Devil and a hard place or whatever the phrase is.

I think the LDO Regs are the way to go .... but still open to suggestions re the source of power.

Andy