MBP15: 45W works only right after unplugging 85W
As far as I know, the 15-inch MacBook Pro is not supposed to use the 45W power adapter.  Therefore, when you plug in the 85W and quickly swap in the 45W, you are essentially tricking the MacBook into thinking that it is still charging from the 85W power source.  

I don't believe this to be dangerous to your MacBook in any way, but it is definitely a hack and it is unlikely that this can be done without using an 85W adapter.