Can a pull down be used to tie a line to ground?
Yes, you can pull a floating line or a pin to ground with a pull-down resistor.  That's what pull-downs are for.

... if nothing else is pulling the line up.

In most microcontroller schematics, the reset pin needs a pull-up to keep the μC running and not resetting.  If you enable an internal pull-down, it will form a voltage divider with the external pull-up.  As a result, it will pull the reset pin to some non-zero voltage.

Many microcontrollers (μC for short) have a mechanism for initiating a reset internally. Such mechanism can be invoked from firmware code.  It preforms the reset regardless of the reset pin state (voltage).

In some μC, it's possible to disconnect the reset function from the pin and use the pin as digital I/O.

There may be a leakage current associated with a floating pin.  (In other words, a floating pin may not be perfectly floating.)  This current sets the upper limit for pull-down (or -up) resistance.  See also a discussion about pull-downs and leakage currents in this thread.

P.S.  Maybe I'm missing your question.  Maybe, some specifics are missing from the question, such as the model of μC and wiring of the reset pin.