If a character has the feat Improved Bull Rush and uses the Hydrolic Push to perform a bull rush maneuver, does he get the +2 CMB?
Is there a general rule to cover such spell/feat combinations?
yes, it works fine... you should also be able to apply the effects of greater bullrush or other such feats.
the spell says 'you... make a bullrush', 'your CMB is...' and the feat says 'you receive a +2 bonus on checks made to bull rush a foe'.
the spell/feat work together by using the general rule, that they wouldn't work together would be an exception to the general rule.
if the spell said the blast of water (i.e. spell effect) ITSELF made the check, you wouldn't get the bonus because YOU aren't making the check, but spell's wording is clear that YOU are making the check. conversely, black tentacles' CMB doesn't change if the caster has improved grapple, because the caster isn't the one making the check, the opposite of the case here.
Good! I wanted to make sure I wasn't missing something when combining those:)