OK! I created a 3 key and a 4 key on a test L5 remote and used the "Full Assign" mode to learn the 3 key and 4 key on my Comcast DVR remote. I then created two macro keys, 333 and 444 using the 3 key and 4 key. If you try the "Full Assign" mode, you will discover that it requires two key presses. Assuming that the number codes from the Comcast DVR remote include a parity bit, that suggests that one key press saved has the parity bit set and the other key press saved has the parity bit clear. Since both key presses are saved, I believe that both are transmitted when I press the 3 key or 4 key created on my L5 remote. If so, the DVR should always respond to one of the two codes.
Originally, I inserted a 1 second delay after the first and second key presses in the 333 and 444 macros. The first time I tested the 333 macro, I got to 100 tries with no error. After playing around with the delays a bit, I started seeing errors. Presently, I have no delay after the first key press and a 0.5 second delay after the second key press. Typically, I see one or two errors out of 100 tries. I don't think that's two bad. The errors may be the result of less than optimum line up between the L5 remote and the IR receiver on the DVR. I usually tilt the L5 remote up a bit rather than pointing it directly at an IR receiver. Another thing to avoid is accidentally pressing a macro button a second time before completion of the processing of the first press.
Now its time to relearn the number keys on my L5 Comcast remote using the "Full Assign" mode so that the parity bit issue is solved when manually pressing the buttons or if they are included in a macro. Let me know how you make out if you try the above. I will upload my Comcast DVR L5 remote as soon as I complete relearning the number keys.