Can you only standby for that segment or for your final destination?

Example:

Currently booked on HOU-LAS-SFO with a change in LAS. The next preceding itinerary for sale on HOU-SFO is HOU-(SAN)-SFO direct routing. If I arrive to HOU to find that the HOU-LAS leg is late can I only standby for an earlier HOU-LAS flight or can I try for the HOU-(SAN)-SFO routing? My objective is to try to get into SFO a little earlier than my booked itinerary but I'm not concerned about connection time in LAS. Also, catching an earlier HOU-LAS flight would still put me on the same LAS-SFO flight.