I've not tried this, nor have I looked at it, so ...
I'm wondering if the data is available in xmltv format. If so, it might be possible to either:
1. Process the xmltv grab to just include this channel data/configure xmltv to only get this one channel, and/or
2. Make sure that this one channel is set to use external EPG (the data above), with everything else set to use OTA
And if the xmltv data doesn't exist for the HD channel but does for the SD one, then you perhaps have the option to sed/awk the grabber file to change the channel name before importing it, or duplicating the SD data under the HD name.
Perhaps none of that makes sense, though, but there might be something there...