When researching this question I came upon a presentation entitled Enabling a Fully Interoperable GNSS Space Service Volume; slides 12, 24, 25, 26 are also shown below. The idea is that while GPS satellites concentrate most of their radiated power at the earth, there is some "spillage" past the edge of the earth and out into space, from both the main lobe and side lobes, depending on the particular GPS satellite configuration.
Signals may be weaker and good fixes may be intermittent, but for many space applications this can still be extremely useful.
This answer has a great discussion (and nice plots!) of radiation patterns.
The last slide suggests the example of a mission returning from the moon, which I assume is a potential future mission.
Apart from the signal strength data from a piggy-backed experiment aboard AO-40 (and here), has GPS ever been actually used or even validated or tested as a potential source of supplemental navigational information beyond Geostationary orbit?
