So in build 23, I liked to play skirmish using the Rift addon and hide my brain in there and just go around blowing ♥♥♥♥ up for the sake of blowing ♥♥♥♥ up. It was fun, I found out that b26 had come out so I downloaded it for all the new goodness it would bring.

Whenever I put the brain inside the rift it will mark it as a dead brain so I can't exactly do this anymore. Is there a mode that allows for playing with no goals, essentially just sandbox mode with heaps of enemies spawning everywhere?

The quickest thing that comes to mind is this:
Open Base.rte\Actors\Brains\Brains.ini;
Go to line 119;
Change this line from

to

Now the regular brain can't be hit by anything, thus indestructible.

Yes, i can totaly agree with that just even doors inside your base make it impossible for your brain to be there.