I also did install multiboot / Nitdroid when i had the stock PR1.2 installed.
The magic of multiboot is that i can return to the stock kernel by just rebooting with the keybord open and pressing B key.
Some days ago i had PR1.2 / Nitroid 0.0.8 and power 40 installed.
As i wanted to upgrade to PR1.3 and just to be on the safe side i installed backupmenu as a multiboot item and made a complete system backup.
After, to upgrade to PR1.3, i just booted to PR1.2 and made an OTA upgrade with no problems. Multiboot nitroid backupmenu and all my other apps were still there and working. At last i replaced the old power40 boot image and modules with version 45.

I think you can install multiboot / nitdroid with power45 installed but then if you reboot with B key you will not return to stock kernel but to power45.

Take a look at the following links that have all the info you may need:

Multiboot/Nitdroid-Autoinstaller

nitdroid forum

Install backupmenu as a multiboot item

My gratitude and credits to multiboot / nitroid developer team, Titan, RobbieThe1st, Rob1n and maemo community.