You cannot get it earlier as a summoner
AP 29 is Mother of Flies (Council of Thieves #5). And you are not required to play the adventure to benefit from that rule.
Hell Hounds are added to the Summon Monster II list of Asmodean priests (clerics and other divine casters who worship the god). The full list of spells added to their list on that book is as follows:
- Summon Monster II: Hell hound (LE)
- Summon Monster IV: Cerberai (LE)
- Summon Monster V: Bearded devil (LE)
This information was later reprinted on the Monster Summoner's Handbook Player Companion (I strongly advise you get a copy of this book), which has a list of all expanded summons for priests of certain deities:
But has restricted access, as noted on page 2:
Clerics who are priests of one of Golarion’s major gods also benefit from having creatures added to their summon monster spells to represent the influence of their specific deity.
And again on page 30:
Priests can use summon monster or summon nature’s ally spells to summon creatures strongly associated with their deities (and often unavailable to other spellcasters). The following creatures are available when a priest of the listed deity casts a summon monster or summon nature’s ally spell of the appropriate level, or uses a higher-level spell to summon multiple creatures from a lower-level summon monster or summon nature’s ally creature list.
As for who can become priests of Asmodeus, Inner Sea Gods has the following entry:
Asmodeus's extremely hierarchical priesthood includes clerics, inquisitors, sorcerers (especially those with the infernal bloodline), conjurers, diabolists (Pathfinder Campaign Setting: Princes of Chaos, Book of the Damned, Vol. 1), fallen paladins, and cruel soldiers. Many scheming bards and vicious monks are also affiliated with his church.
As you can see, summoners are not part of Asmodean church, nor they get divine powers from worshipping Asmodeus. As such, they cannot gain the benefits of being a "priest", which includes several things on top of being able to summon hell hounds sooner.
However, as noted by one of the PFS Global Coordinators, a summoner could qualify for traits (like Contract Master) and feats (like Devilish Pride) restricted to worshippers a specific deity if they note that down on your character sheet, and meet the necessary requirements. But the restricted summons are not mentioned.