XSPC are a good example. Their acrylic res' (the ones I've used in the past) have a larger fill port with a plastic screw in cover that's supposed to be opened with a coin. I think Bitspower have come up with a fitting to open up new possibilities like Zia's.
I have done it numerous times. Here are two with the top used as an inlet...

Still gave a nice clean cut that didn't need much cleaning up.
