HexRay API - Programatically rebaseing program on firmware

I am working on several firmwares (mostly ARM and PPC) that have no structure that IDA can decipher and so far I have been guessing BASE address until the functions and strings align with what I see in the header.

I want to write a plugin in C (preferably) that can reset the rebase of the current executable that I have open and possibly mark the first few lines in the image as function pointers (pressing Q) .

Is there an API call for IDA to readjust programatically the BASE address?