From an executional standpoint, it helps to know how the harp's mechanism works.
The harp's pedals are laid into a zig-zag pathway, and have the same kind of spring tension in an upwards vertical direction as a piano pedal. However, the pedals can also be moved side-to-side to go through this pathway. When the pedal is all the way up, it can be pressed vertically down and horizontally away from the center of the instrument to rest on the next notch in the zig-zag. From here, there is one more notch where the pedal can rest, again, by pressing down and away.
The harp's strings are anchored to a tuning peg at the top of the instrument, and can be altered up in pitch by half steps by small rotational mechanisms that grab the strings between two pegs. There are two of these mechanisms per string, allowing for a range of two half-steps of alteration.
Each pedal corresponds to a set of strings on the harp (all the same pitch in multiple octaves). When the pedal is all the way up, the alteration mechanisms are both open, so the full string is vibrating all the way to the tuning peg. Moving the pedal down to the first notch actuates the first mechanism, which rotates to grasp the string between two pegs at a half step. Essentially, it's like a fret, except instead of pressing the string over the fret, the fret is grabbing the string. The second notch on the pedal actuates the second mechanism, raising the pitch of those strings a second half step.
So then, the half pedal technique involves only rotating the peg mechanism (tuning gear) so that it comes in contact with the vibrating string, but doesn't shorten the vibrating length. It would be like striking a note on the guitar and then lightly brushing your fingernail on the string without dampening it fully. The harpist would execute this by moving the pedal vertically until the gear starts touching the string, but not so far that it can be rested on the next notch. Since this is going to be different for each harp, this definitely constitutes an "extended technique", and I concur with @jjmusicnotes that the sound of this will be very subtle. I'll suggest it might best be used in a solo harp setting.