The factors affecting whether UA holds a flight are many and you should not rely on it happening. # of pax certainly helps, UA-to-UA, last flight of day, etc. But, also, ATC, fuel, crew times and turn at the other end (if headwinds will delay your flight, UA doesn't want to delay it further), onward connections for pax affected (what if holding for you causes 50 pax to misconnect at other end). Lastly, a negative for you is that if UA needs your F seat (is it actually paid full F?) for somebody else and is actually hoping that somebody misconnects so they can legally acommodate the other person.

For all of these reasons, you should not attempt this unless you have: 1) backup plans locked in; and 2) understand that you may need to do this on your own with no help. Of course, it is of physical assistance that you are in F and can make arrangements to make a run for it.