I found a way to add a "30 Second Skip forward" for a Comcast Motoroal HD-DVR. I just tried this on my Motorola DCH3416 and it does work.

You'll need the silver original remote that came with the cable box to get it to work.

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The following technique can be used to map an unused or unneeded button on the "silver" remote to the 30-second skip command. Current versions of the i-Guide software will skip forward 30 seconds into a recording when this command is sent. A good choice is the 'A / Lock' button since many users don't need that function; you can feel both the '15-second-back' and '30-second-skip' buttons with one finger and move between them without looking. Another option is to reprogram the '15-second back' button, since PgDn already provides that functionality.

1) Press the "Cable" button at the top of the remote to put it into Cable Box control mode.

2) Press and hold the "Setup" button until the "Cable" button blinks twice.
Type in the code 994. The "Cable" button will blink twice.

3) Press (do not hold) the "Setup" button.

4) Type in the code 00173.

5) Press whatever button you want to map the 30-second skip command to (ex: A / Lock). The "Cable" button will blink twice if successful.

Note: This does not work on boxes with the Microsoft TV Foundation Interface (previously used in Washington State, but now converted back to the i-Guide software). As of firmware version 12.22 Insight Cable disabled this feature, but has been re-enabled in firmware version 16.35. Comcast software Version 73.44 disabled this feature, but the latest software, Version 74.53-3321, re-enables it (this has been confirmed on the 3416 w/ 16.20 firmware). See software for more info.