Yes, in many cases, you can use the heart rate monitor capabilities of your GPS device if you are not within range of the GPS signal.
In particular, we've tested both the Timex Data Recorder and the Garmin HRMs and they both work.
Since early last year, we have been using a special dataPt element (described in our extensions) that allows us to put information such as heart rate, temperature, altitude, cadence, etc. into the GPX file when GPS coverage isn't available. As a matter of fact, when the GPS claims that it sees no satellites (even in the middle of a track), we put the additional data out using dataPt elements so that we don't face the choice of dropping data on the floor or faking it.
The next step is getting the data interpretation software to support it. For the time being, we believe that TrailRunner is one of the few programs that will correctly interpret this data.