I'm not familiar with upgrade availability to Central America, but some general tips:

1) Start by using expert mode to search *only* for the legs you want to upgrade to business. Look for PZ in those buckets. For example, for YVR-SFO-NRT, search only for SFO-NRT, not YVR-SFO. This is because United's search results will only show you fare buckets that are available for all the segments (generally; in some cases, the segments aren't married, so you will see segment-by-segment availability). This is because PZ is generally unavailable on domestic/shorter segments on UA, so to see PZ availability on the long-haul leg, you have to search just for that leg.
2) Be prepared to be flexible on dates. You may have to travel on days that may be less convenient for you or have to shorten/extend your trip to get the GPUs.
3) As a reminder, to upgrade to business using a GPU, you have to buy an economy fare of "W" class or higher; in many cases, this will require that you pay more than the lowest available economy fare. (Correction: as jsloan reminded me below, this only applies to routes where a GPU is required for an upgrade. For routes where an RPU can be applied for an upgrade, any economy fare class will work.)

IIRC, none of UA's flights to Central America have lie-flat seats (and just aren't that long, especially once you get to the connecting point). If you want to use them for lie-flat seats, you'll likely have to consider Europe or Asia.