Actually, CO has at least one Friday flight from IAH on B738. (There is also mainline service to GDL at least once a day, and probably other mexican cities as well, but anyways...)

I travel regularly to GDL. The trick is in the scheduling of flights. Of course, you need to book a flight on non-ERJ to have any chance of upgrade. Weekend flights from IAH tend to have a much lower business component and a much higher proportion of cost-consious travellers, and are thus better for upgrades if you are Elite.

Even as NW Silver, I score fairly regularly on CO when traveling off "business-travel times". All my CO upgrades have been at the gate, despite often being confirmed for upgrades on the NW segments. I've never been upgraded by CO when not upgraded on the previous NW segment.