Andy’s Effects (free), BCC10 , Boinx Software, FxFactory, motionVFX, Industrial Revolution, Universe, Sapphire, Luca’s Random Text, Film Impact Bounce Pack, Core Melt….and i’m sure many more but these are the ones i use.
They will ignore your request as they have my 5 requests for a fix to this. It’s possible that it can’t be done with the current state of their software design. If it’s not possible then i wish they would tell us.
I too came from FCP7 and i can say that really hate how Premiere handles this. i tried nesting and it’s a crap way of doing things. so do what you described and on your “fill” clip apply “Andy’s Better 3D” ABOVE track matte key. things will jump around a bit maybe. position your matte where you want it. you can even resize it. now using Andy’s Better 3D reposition and resize your clip. Andy’s doesnt have a size control so you will have to use his Z Position to resize you clip. it’s goofy but it works. Good Luck. and to Adobe i give you the one finger salute.
ps Andy’s is a free download plugin. if you have Boris Perspective Unit then that works too but better but it’s not free.