can the engine be modified so the player can ...

[hondobondo]

move upside down?
I want to run through loops like this one:

or is there already an engine that can do this? obviously the player speed must be modified in qc

[Baker]

I wish. But the answer is no.

Would be an all-time killer feature.

The topic comes up from time to time often in the form of "it is possible to make a walk-on-walls mod, etc." or "walk on the ceiling".

FTE has some of the pre-cursors to handle some of the challenges involved, but AFAIK even with FTE is isn't even close to possible.

[Spike]

try fte's 'spiderpig' console command some time.

logically any engine with tracebox could be tweaked to do it, but you'd have issues with player angles - yay csqc as a way around that.
it'd be a lot of work though.
if you wanted to do it properly, you'd need to use some sort of spring for velocities with conservation of momentum, rather than directly clipping the velocity and then falling away instantly.

[Baker]

try fte's 'spiderpig' console command some time.

Simpsons Movie reference

[frag.machine]

IIRC the original Carmaggedon had at least one loop track like this.
I wonder if vectorial gravity would do the trick.